Where can we get info on the number of Bitcoin nodes in use? That sounds like a great metric.
It's not really that great because it doesn't account for all the tens of thousands of online wallets secured by exchanges and ewallet service providers.
Also, unless it's something like "different nodes seen in the last week" it wouldn't account for people like me who only run the Bitcoin client long enough to catch up on the blockchain and the occasional transaction.
This whole forum is full of statisticians that will tell you how to account for outliers.