I still don't understand that 250k figure. Number of active BOINC crunchers (active=returned a task within last 24 hrs) was usually around half a mil (and increasing slowly). Now it's almost back to that number: 430 892, according to the latest stats on BOINC front page.
But I am aware it was around 250k until recently. For example, Archive.org snapshot, taken exactly three weeks ago, shows 270k active crunchers. I don't think we have gained 160k new crunchers in only three weeks, it's more likely that we are simply seeing correct stats again.
The quantity of active BOINC users fluctuates and varies depending on what website you visit (different on netsoft vs boincstats & I'm sure freedc shows something else too).
250k is a conservative guesstimate, still a potentially massive userbase to recruit!