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 think the best way to judge adoption would be market value.  BitCoin is vastly inferior to credit/debit cards when it comes to store purchases, and stores that accept it are doing it for novelty purposes.   Google is probably not that great either, as most of the articles on BitCoin are about how it's about to die has died 100 times in the past.

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).

market value
merchant adoption

the other two are scewed as i know somepeople have more then one blockchain wallet and some people use google to find pages, instead of book marking them.