Are you implying that all people who are invited to the event have a Facebook account? Do you find it reasonable to require them to have one? Maybe you do, but not everyone does.
I'm not trying to imply that. Yes, I do think it's reasonable to require a Facebook account for college students. We also publicize the events in local papers, flyers, word of mouth, and even our blog. Facebook is the best way to get >85-90% of students who come to come.