Yes, what you said. Bitcoin is just getting started, so it seems reasonable to think that the slope from "none" to "some" might be both bumpy and steep. Does anybody know of a good source of data for the exchange rates of other currencies when they were in their infancies? That could make an interesting comparison.

I don't know whether bitcoin is in a bubble or not (it does seem likely), but the rising slope of a sigmoid graph can also look exponential until you get out further in time.