Their goal is of course two fold, learn as much as possible about you (they are already doing good job here) and earn as much money as possible. So what could be reason for looking at crypto currencies world? Probably to implement some kind of payment system like paypal into FB.
First step would be to easy send money to another user, second step to start being some kind an Amazon a little bit, a store and getting a cut from every transactions, product advertising, and of course intelligently connect users with products based on users profile...

Make more money, that is my guess. "...let's cash in better our huge user base..."