It seems like there are cheaper ways to improve their users' experience on existing mobile platforms.
The only non-nefarious reason I can think of is they feel a threat from both Google and Apple in their area of expertise - social graphs.
Of course, it's possible that it's all a feint on the part of FB to get some loving from Apple.
http://www.telegraph.co.uk/finance/newsbysector/mediatechnol...