I'm pretty sure my own kid knows that I'm kidding her on her page. She has said, "The only time I drink is when you offer it to me." And I had said that we should get her drunk safely at home before she goes, so she'd have a sense of how the process works and feels in her own body.
The other kids might not realize it's a joke, so that's why I suddenly stopped. In fact, I think I'm going to have a policy of not commenting on any posts that her college friends make, even if I can see them, etc.