https://en.bitcoin.it/wiki/Bitcoin_Days_Destroyed
Here's a chart of BDD per week (log scale is best in this case):
http://blockchain.info/charts/bitcoin-days-destroyed?showDat...
Here's an attempt to track transaction volume, minus change transactions (the Bitcoin client almost always performs a transaction to issue change back to the sender whenever he/she sends money to someone): http://blockchain.info/charts/estimated-transaction-volume-u...
Both of these metrics point to a slow but steady growth in the number of Bitcoin transactions conducted -- this in spite of a huge increase in their exchange value against the USD and other fiat currencies. So people are spending their Bitcoins. And this trend will likely continue as more companies accept Bitcoins.