My friend sent me this: a Cow Clicker game. This is absolutely brilliant parody because it encompasses everything strange about the "social Facebook games".
Now, back to the subject - how can Facebook earn extra millions ? I think in the same way. They can use their "Like" button instead. Humans want to be heard. They especially dislike when some other humans are heard and they are not heard. They consume the friend feed in bursts (unless facebook is the only activity they do).
So, the solution is to limit the number of feedback/click items per hour - better, per the interval of something inconvenient like 5 hours. Sell the Facebucks and allow to use those Facebucks to pay for the ability to express themselves here and now.
Tada! Instant revenue stream!
Of course it'd be evil beyond belief.
And presumably the "likes" that help to build the semantic network are more valuable than the money buying them would bring.
But this could be a fun experiment.
Not talking of the fact that having the Facebucks would be a no-brainer side biz for micropayments.