You should use bitcoins to buy drugs on silk road using tor. The rising popularity of silk road can be seen in the exchange rates for bitcoins in the last few months.
Let's say it takes a week between when a person buys bitcoin and when those coins are spent.
That mean about half a million dollars of currency is tied up for these users. That's about 50K BTC. That's less than one week's production from mining.
This marketplace has almost no impact on the exchange rate directly. What it might be doing is introducing a number of people to bitcoin. They might then speculate on the exchange rate (simply by buying more than they plan to use for purchases, in anticipation of the exchange rate rising), or use bitcoins for purchases or for person-to-person payments.
