Total output volume may be a useful metric, since people may have used a Bitcoin transaction to move Bitcoin from one exchange to another. But a large amount of Bitcoin can be sent in a single transaction.

https://blockchain.info/charts/output-volume

Another popular metric, since output volume can be pumped up by sending the same coins over and over gain, is Bitcoin Days Destroyed.

https://blockchain.info/charts/bitcoin-days-destroyed-cumula...

http://bitcoin.stackexchange.com/questions/845/what-are-bitc...

Also transactions on the exchanges don't show up in the blockchain.