You can see how many Bitcoin clients are being run:http://bitcoinstatus.rowit.co.uk/
Of course one doesn't need to run client software in order to have or use Bitcoin. Your bitcoins can be in an exchange or online wallet, or your wallet can be safely stashed away for a rainy day.
IMHO, the number of full clients correlates only negatively to the chain size and does not correlate at all to the number of users (because of hosted wallets like paytunia).
The number of users would include people using bitcoin in at least one of the three classical functions of money (store of value, unit of account, medium of exchange).
For instance a merchant accepting bitcoins would not even need to use it in any of the other two functions (accepting through a service provider like bitcoin-central then cashing out to fiat at the end of the day).
Yet the merchant would be using bitcoin as a unit of account (displaying prices in bitcoin) and helping promote the use of bitcoin as a means of payment.