Network effects. As long as the other kids socialize on social media, not using social media becomes a drastic socialization penalty. The problem cannot be solved individually, only by collective action. Most parents must ban social media (and electronics in general) for their kids. Which indicates a need to pass laws to ban social media for minors. Bans and teens don't usually work as intended, we probably also need a wide campaign of stigmatization of said social media, akin to anti-smoking campaigns.
plus many schools etc are using facebook etc to disseminate news... and many kids friend prefers to communicate in facebook groups. Many teachers are also available in facebook.