Also the number of transactions per se is not the best measure of bitcoin's importance; it is the total purchasing power being transferred that should be considered. Bitcoin is a medium of savings that allows people to transfer value easily, from any point on the globe to any other point with minimal fees. The total purchasing power being transferred has growth tremendously because BOTH the number of transactions has grown AND the purchasing power of bitcoin has increased manyfold in the last few years.
This, of course, does not even factor in all the transactions that happen off blockchain.
I think looking at transactions is a reasonable metric. In fact, even this metric may be overstating things. Tim Swanson has analyzed transactions excluding popular addresses (apparently this excludes things like SatoshiDice) to identify transactions that are positive-sum in an economic sense and then the numbers look even lower. [1]
Another factor to consider is that if you count purchasing power transferred (which I assume means dollar value times transactions) you'll end up giving a lot of weight to events like when coinbase/mtgox moved their coins from one wallet to another.
Here he compares bitcoin to m-pesa which is a deeply misleading comparison. The former has a floating price, the latter is a money substitute and its price is fixed against the currency which is substitutes for. This is why I've suggested to Tim that it is total purchasing power transferred that should be measured, and I think I have convinced him of that.
This, of course, does not even factor in all the transactions that happen off blockchain.