quick summary: after winning the identity war, launch a third-party ad service. if it happens, Ryan projects FB could be worth $100B
One of the main arguments against FB monetizing their ads is because "FB users don't go on FB to click on ads". This is anecdotally true. But when FB Connect and Like buttons have you logged in everywhere, there's nothing stopping them from showing you targeted ads when you are shopping, or searching or doing things where ad CTR would be much higher.