Are you sure you’re not signing up for Facebook Mobile? Because in order to do that, you need to send in your phone number and get a verification text. my brother signed up for FB recently and they just had to verify the email, nothing more. If you’re worried though, I can assure you, Facebook is good with your number and won’t do anything “funky”. I’ve used facebook Mobile and it didn’t cost me anything nor was I sent “junk texts”.