On Facebook there's this guy who said 'who what's to be my girlfriend?' and I really like but I don't know what to say!

So he will make whoever answers his girlfriend? Sounds like he isn't very picky about his women. Ewww. Say nothing!!

Do you actually know this guy?
Why would you say anything to a guy who just puts it out there and doesn't care who picks it up?

Sounds to me like he's either 12 years old, or plain immature. You sure you like this fool? Maybe try and get to know him first.

I'm not trying to be mean but it is possible he was just joking.