PhoneBomber is doing pretty good - heading to 25,000 users. At any given time there's over 10,000 bombs queued up - and there have been over 1 million placed. Bunch of people look very active, and ALL over the place.
You can see details over at http://www.phonebomber.com/
Here's a map of all bombs that have blown up since the end of last year (ouch!)