Serious answer? It's a distraction from work, which could mean they flunk out of their courses. But if you're addicted enough to Facebook for it to affect your studies then you're looking at bigger problems than not graduating. Another reason it could be bad is that getting drunk at parties, posing for stupid photos and everything that you do when you get that first flush of freedom - that's all easily documented now, and a breach of discipline is saved for posterity now - which can mean expulsion from college or missing out on jobs if a future employer sees it.