I think the real problems will start for Facebook when advertisers figure out Google gives them much more bang for their buck.
Look up the term “Google slap” and then decide how much “bang for the buck” Google provides. Their content network is a joke as well. Advertisers are really a little tired of Google and their strong-arm tactics with Adwords. I’m not going to predict Facebooks stock value, but there is plenty of opportunity for them to expand their advertising revenue.