Would it be weird to facebook a guy I hardly know?

Probably not, it might make you look ditzy and desperate though. It would not be weird, because a lot of girls who use facebook are ditzy and desperate.

It doesn't have to look that way, it depends on how you do it. If you add him, message him AS PLANNED and explain "hey, I got your facebook, and I thought it would be interesting to talk with you online"

There you go.

Let me reflect this question back to you in this way.

Is it weirder to have a crush on a guy and remain infatuated with him but never talk to him about it

or

is it weirder to facebook a guy that you've met, and hung out with and are just a little curious about him?

everyone is afraid of rejection, but then again, you may never know for sure.

ask for something casual like, hanging out at your friends house, inviting him to parties.something social, not one-on-one. and if he constantly dodges you, than its a hint that he's not interested, but if he wants to, than you're in luck.

either way, my point is to "test the waters" and see if he bites.

go for it, facebook that dude. the worst than can happen is for him to say, "yeah, uh, I actually am doing something else that night."

umm since you said to facebook him, I thought you meant to add him as a friend on facebook. And there's absolutely nothing wrong with that. I've had people add me as friends or I sent out a friend request to someone I met the night before at the bar and we talked for a few minutes and went on our way.

But he might be creeped out a bit if you msg him saying that you have a crush on him, and honestly could you blame him? I agree with evilsheepempire that you should take it slow. Add him as a friend first, then chat a bit and ask him to hang out.
Good luck!