I agree. One market I'd like to see them enter is online payments and micropayments, they have enough reach that they could probably pull it off.
I'd like to be able to easily send $5 to a friend via facebook, or 25 cents to the author of a blog post I enjoyed. Others have tried the micropayments, but it needs a heavyweight to really pull it off. Maybe facebook can buy a startup that has it nicely figured out technically, but can't get traction.
But it's still the beginnings of an entry into the market.