It it weird to tell a girl that I might add her on Facebook?

It's weird to tell ANYBODY that you 'might' add them to your fb.

It makes you appear pompous.

Kinda depends on how you said it. If she or you was about to head out and you quickly asked "Hey, are you on Facebook? I'd like to stay in touch." Then it wouldn't be awkward. But if she was talking about a funny joke her stylist told her last week when she was getting a perm and you said that you would add her on Facebook out of the blue; it could be perceived as a little strange.

Yeah,bro
akward isn't even close.