By adoption do you mean, people buying and holding, or the velocity of BTC in the economy excluding transactions between owners' addresses?
The former can be measured by the price over time. The latter, I'm not so sure about. Some things spring to mind:
* Total VC funding received
* Number of new bitcoin related projects, and total number of bitcoin projects
* Active commits to bitcoin projects on Github
* Increasing hash rate means more people are investing in mining (they think adoption will happen)
* Number of bitpay users over time
I'm sure there are plenty of other more meaningful metrics. I think it's hard to deduce just from the blockchain because you don't know if people are moving BTC around between their own addresses.
I suppose what I am interested in is a single metric that best captures overall global interest from consumers and investors. My personal favourite is the number of My Wallet accounts on Blockchain.info (currently standing at over 1.5 million and increasing by 6,000 a day).