Facebook is a homepage where one can share photos, status updates about their life etc.
Fb is not something one says while speaking, it is an internet chat or sms abbreviation.
"I uploaded all the photos from last night on Fb!"
1. "fucking bitch"; usually used as a curse/exclamation. Sometimes used as a hashtag by users on Twitter, www.twitter.com
2. Social networking site Facebook, www.facebook.com
Ted: "Stubbed my toe on the corner of my desk >:( #fb"
Rob: "Hey man, are you on fb? You should add me."
Ted: "Whoa, hey, did you see Rob's profile pic on fb?"