If bitcoin really catches on, if it becomes facebook popular, it will have about 1 billion users. The final number of bitcoin is 21 million, making 21 billion millibitcoins. This means that there will only be 21 millibits per person. This is too little. A person will want a good number of units to feel good. I think a good target is to aim for 1000 units for people with not so much bitcoin. The poorest 50% will only have 1% of the currency, so we need
5 billion x 1000 for the poorest, and that is only 1% of everything. So we need 500 trillion = 5*10^14 units of currency. A close unit to that is actually satoshi, of which there are 2.1*10^15.

Therefore it might be that we need to plan for satoshi as a very long term unit.