Social networking sites (Facebook, MySpace, YouTube - launched 2005) are neutral communication tools with no significant psychological effects.

When Facebook expanded beyond university campuses in September 2006, it had twelve million users and no entry on any school's list of student health concerns. Guidance counselors thinking about adolescent wellbeing in 2006 were focused on the same risks that had occupied them for decades: substance use, eating disorders, unsafe sex, and bullying, the latter understood primarily as a physical or verbal phenomenon occurring in hallways and on school grounds. The idea that a social networking website might constitute an independent risk factor for depression or anxiety had no research basis, because the phenomenon was too new to have been studied. It also seemed implausible: these were websites where teenagers shared photos and talked to friends. The concern that television rotted brains had faded into parental folklore. A website seemed less worrying.

Research began accumulating after social media use reached mass penetration, roughly after 2012, when smartphone ownership among American adolescents crossed 50 percent and social media use shifted from a desktop activity to a constant ambient presence. The pattern that emerged in the data was not subtle. Jean Twenge at San Diego State University documented sharp increases in rates of depression, loneliness, and suicide-related behaviors among American adolescents beginning around 2012, with the inflection corresponding closely in timing to the smartphone and social media transition. Girls, who used social media at higher rates and were more frequently targeted by social comparison content, showed larger effects than boys.

The causal mechanisms proposed were multiple. Passive scrolling through curated images of peers, selectively presenting highlights, attractive self-presentation, and evidence of social inclusion, drove upward social comparison, the tendency to evaluate oneself against an unfavorable standard. Algorithmic feeds designed to maximize engagement amplified content generating strong emotional responses, outrage, envy, distress, at the cost of mood. The displacement of time previously spent sleeping, exercising, or in face-to-face interaction shifted behavioral patterns documented to affect mental health.

Jonathan Haidt and Twenge's collaborative review, synthesizing hundreds of studies across multiple countries, found consistent patterns linking heavy social media use, particularly among adolescent girls, to increased anxiety, depression, and loneliness. Similar trends appeared in UK and Canadian data, suggesting a pattern that wasn't specific to American cultural circumstances.

The evidence is not fully settled. Some researchers argue that the association between social media use and poor mental health is weaker than advocates claim, or that causality runs partly in the opposite direction, that depressed teenagers use social media more, rather than social media making teenagers depressed. Studies that rely on self-reported use tend to overestimate consumption. Experimental designs are difficult to conduct because randomly assigning teenagers to avoid social media for extended periods presents both ethical and practical challenges.

What is clear is that the early framing, these are neutral communication tools, essentially phone calls made faster, failed to anticipate the scale of the effect and the specific features, algorithmic amplification and social comparison, that distinguish social media from earlier communication technologies. In 2006, uncertainty was reasonable. By 2018, uncertainty about the basic direction of the effect had become difficult to sustain in the face of consistent findings across multiple countries, data sources, and research designs.