Agreed that anonymity brings trolls. What about low-friction sharing that Facebook Auth brings? How did this play into your decision? You said earlier that you used to ask for that authorization AFTER asking for login auth?
Yes, asking for FB permissions twice was tripping over our users however. What seems to work right now: login with Twitter or username/password, and add FB auth later when a user wants to share. This apparently doesn't feel the same as allowing FB on signup. Makes sense?