I just can't see bitcoin taking off in a big way, when it is capped at 21 million. Imagine 21 million divided between the facebook population (lets leave country and nationality out of it) 500 million or half a billion users . That's not a lot of bitcoins to go round, it won't buy you a lot.
This is a FAQ (or "FMC", frequently misunderstood concept?).
1) Bitcoins can be divided into millionths of a bitcoin, leaving plenty for everyone on Earth.
2) "it won't buy you a lot" There is no economic reason why 0.000001 BTC cannot be worth $1,000 USD. It is entirely supply and demand.
Where you place the decimal is completely irrelevant.