For Facebook (the NSA's friendly right arm), to track Bitcoin transactions and addresses.
The whole point of BTC is to avoid that....
DUH !!
When you sign up for MtGox or any of the other exchanges, you have to hand in bank statements, utility bills and government issued ID to people you've never met before. You don't think the NSA can see that?
FWC just needs to know if you're a friend of the seller. And Facebook Connect does not mean the site has to give Facebook access to their database. They can't see what's going on within the site. It's just used for authentication to see if you and the seller/buyer are friends.