I can't really pick one over the other, because they both have their pros and cons.

TFF is good because I can't go into my Facebook account and just randomly starting up a conversation about Final Fantasy, and expect someone to reply to it, with the exception of one or two people.

Facebook on the other hand has all of the people I know in person, and you can talk with them with greater ease then on TFF most of the time, because they know you.