Comment putting on my tin foil hat for a moment... (Score 4, Informative) 185
There's more to it than meets the eye. I don't have a FB account, so I can't fathom why they would ask for you to include your phone number on it for any reason. I do know that Google now REQUIRES it just to open a Gmail account.
Some part of me simply doesn't trust this. We all know about correlation engines and how they work, and we know that the NSA collects and reads your emails (http://www.guardian.co.uk/technology/2012/sep/15/data-whistleblower-constitutional-rights). Now we add into the mix your phone number, which, as we already know is subject to warrant-less tapping (http://www.businessinsider.com/senate-renews-controversial-law-which-allows-warrantless-wiretapping-of-us-citizens-2012-12) and if the number you provides happens to belong to your cellphone, we know that it can act as a covert "roving bug" (http://yro.slashdot.org/story/06/12/02/0415209/fbi-taps-cell-phone-microphones-in-mafia-case). All of this provides more data to track you, what you do, who you interact with, who you're near at any given moment and those individuals interactions... All in the name of "keeping you/this country safe".
This simply doesn't sit well with me.