This argument is popular but doesn't hold water. If we suppose for the sake of argument that social media is bad for kids, then it is a parent's duty to keep them off it even if that makes the kid unpopular. As an analogy, consider a kid whose social circle is all shooting heroin. Would it then become acceptable to let your kid do heroin? Of course not. Similarly, even if all the other kids are on social media, it doesn't become acceptable to let your kid use social media (going with the assumed premise that it's bad for them).