The increasing use of social media has sparked significant interest in its impact on mental health and well-being. While social media data offers a rich source for understanding various aspects of human behavior, including mental health, there is no universally accepted definition of "social media." This study defines social media broadly as internet-based platforms enabling user-generated content exchange. The vast amount of data and widespread adoption have led to two primary research areas: predicting mental health from online data and studying social media's influence on mental health, particularly in children and young people. Previous research has examined diverse psychological outcomes, including suicide prediction, the influence on disordered eating and self-harm, the impact of cyberbullying, and online support networks. A crucial aspect often overlooked is the reciprocal relationship between mental health and social media use; individuals' pre-existing mental health conditions might influence their social media behavior, and conversely, social media use can affect mental health. Many studies assume a causal link from social media use to harm, neglecting the potential benefits (e.g., peer support, connecting with loved ones) and focusing primarily on pathology. Further, significant challenges exist, such as a lack of comprehensive data on the characteristics of the entire population of social media users. Most existing data relies on convenience sampling or algorithmic predictions, both of which may introduce significant bias and limit generalizability. This study aims to address this gap by analyzing a representative UK population cohort to describe social media usage patterns, demographics, and mental health and well-being outcomes among young adults. The study uses a comprehensive set of well-being measures, going beyond indicators of pathology to consider positive well-being outcomes. The study's primary goal is to better characterize social media users and inform interpretations of past and future research.

The existing literature on the relationship between social media use and mental health is extensive but often contradictory. Some studies suggest a causal link between social media use and negative mental health outcomes, such as increased rates of depression, anxiety, and self-harm, particularly among adolescents and young adults. These studies often focus on specific negative aspects of social media, such as cyberbullying, exposure to idealized body images, and social comparison. However, other research challenges these findings, suggesting that the relationship is more complex and nuanced. These studies highlight the potential benefits of social media, including increased social connection, access to support networks, and opportunities for self-expression. Furthermore, there is a growing body of research that emphasizes the importance of considering individual differences and the context of social media use when assessing its impact on mental health. Some studies have found that moderate social media use can be beneficial, while excessive or problematic use can be detrimental to well-being. The lack of population-representative data on social media users is a major limitation in the field. Most studies rely on convenience samples or methods that may not accurately reflect the demographics and characteristics of the broader social media user population. This can lead to biased findings and limit the generalizability of research results.

This study utilized data from the Avon Longitudinal Study of Parents and Children (ALSPAC), a population-based cohort study that has been tracking the health and development of a large group of children in Avon, UK, since their birth in the early 1990s. The current analysis used a subsample of 4083 participants who completed a self-report questionnaire at a mean age of 24 years in 2016/17. A smaller subsample of 2991 participants also completed a well-being questionnaire a year earlier. The questionnaires assessed a range of demographic factors (sex, ethnicity, education level, parental occupation), social media use patterns (platform use, frequency of use), and mental health and well-being outcomes. Mental health indicators included depressive symptoms (using the short Mood and Feelings Questionnaire, MFQ), suicidal thoughts, intentional self-harm, and disordered eating. Well-being was measured using seven questionnaires: the Warwick Edinburgh Mental Well-being Scale (WEMWBS), the Satisfaction with Life Scale, the Subjective Happiness Scale, the Gratitude Questionnaire, the Life Orientation Test (LOT-R), and the Meaning in Life questionnaire (assessing presence and search for meaning). The Basic Psychological Needs (BPN) questionnaire assessed autonomy, competence, and relatedness. Missing data were handled using imputation techniques. Data analyses involved descriptive statistics (percentages, means, standard deviations), chi-squared tests to examine demographic differences in social media use, and visualization techniques to explore patterns in the data. The analysis is stratified by sex because of known gender differences in social media use and mental health outcomes.

The study revealed several key findings regarding social media use, demographics, and mental health outcomes among young adults. First, the frequency of social media use varied significantly across participants, with over half reporting using social media between 2 and 10 times per day, and approximately 39% using it more than 10 times daily. Significant differences in social media use frequency emerged between genders: women were more likely than men to report higher-frequency social media use. Facebook was the most popular platform, with a high percentage of users. The use of other platforms (Twitter, Instagram, Snapchat, and YouTube) showed substantial differences across genders. Instagram and Snapchat displayed greater use by females daily and overall. Conversely, YouTube use was higher among males. Concerning mental health, results suggest a non-linear (potentially quadratic) relationship between social media use frequency and some mental health outcomes, partially supporting the 'Goldilocks hypothesis' that moderate use is optimal. This pattern was not consistent across all outcomes. Specifically, among women, lower levels of social media use were associated with higher rates of disordered eating, self-harm, and suicidal thoughts. However, these patterns varied for males and different measures. In terms of platform usage and mental health, among daily users, YouTube was associated with the highest prevalence of negative mental health outcomes, such as depression, disordered eating, self-harm, and suicidal thoughts, particularly among women. In contrast, daily users of Instagram and Snapchat tended to report higher levels of well-being across several measures.

The findings of this study offer valuable insights into the complex relationship between social media use, demographics, and mental health outcomes among young adults. The non-linear relationships observed between social media use frequency and mental health indicators challenge the simplistic assumption of a direct, linear relationship. The variation in platform usage patterns between sexes also highlights the importance of considering gender when studying social media's impact on mental health. The significant differences in mental health and well-being among users of various platforms underscore the importance of moving beyond a generalized concept of "social media" and examining the specific characteristics of each platform. The higher prevalence of negative mental health outcomes among YouTube users may stem from the platform's passive content consumption model versus more interactive platforms. The higher levels of well-being among users of Instagram and Snapchat, which are more geared toward social interaction, corroborate the importance of relatedness and social connection for mental well-being. The study's limitations (sample demographics, measurement methods) should be considered when interpreting the findings.

This study provides a valuable population-based profile of social media users and their mental health, revealing non-linear relationships between social media use frequency and mental health outcomes and significant differences across platforms. The findings emphasize the need for future research to consider gender, platform-specific effects, diverse well-being measures, and more nuanced methods for measuring social media use, potentially incorporating longitudinal designs and objective measures. This work encourages a move away from simplistic generalizations about social media's impact on mental health and calls for a more contextualized and comprehensive understanding of this multifaceted relationship.

The study's reliance on self-reported data for social media use and mental health outcomes may introduce biases and inaccuracies. The sample, although drawn from a large population-based cohort study, may not perfectly represent the diversity of the entire UK population, particularly in terms of ethnicity and socioeconomic status. The cross-sectional nature of the study limits the ability to draw causal inferences regarding the direction of the relationship between social media use and mental health outcomes. Finally, the reliance on specific well-being measures might affect the generalizability of the findings, as different measures may capture different aspects of well-being. Further research utilizing objective measures of social media usage, a more diverse sample, and longitudinal designs would help to address these limitations.