We don't actually have a reliable way to measure the number of users. The Metcalfe Value plots I prepare assume that it's correlated with the number of TXs per day and the number of unique addresses used per day.
Do we guesstimate the number of users from client/wallet downloads, key site (BTCT/blockchain) traffic, and Google search trends? Or what?
No we just use the number of transactions per day and the number of unique addresses used per day as proxies for the number of users. It is highly likely they are strongly correlated.