Don’t tell you kid to make their Facebook/Instagram/Vine/Twitter private. All that does is hide the problem.
Instead, insist on them keeping those public. It removes the false security that making those accounts “private” actually prevents bad things from happening. If those accounts are kept public, there’s no illusion: things posted will be seen. By you. By grandparents. By college admissions officers. And that knowledge changes the behavior instead of hiding it.