Statistics about crtversion.txt accesses. This means that a user enabled automatic search for updates from the FET menu and started FET.
On the last ~4.5 months (from 1 December 2011 to 13 April 2012).
1148 unique IP's, 3832 total hits.
Do not forget that some IP's are dynamic. So, we may end up with about 1000 unique persons.
I estimate that less than 10% of the users activate this option (which is inactive by default).
So, there are maybe at least 10,000 FET users in this interval.