The Big Bang ''Hot Time'' Event

If you look at the bottom of the Nexon forums you will see:
"There are 9,237 guest(s) online. There are 358 of 408,472 member(s) online"
Look at the other forums, eg Mabinobi or Combat Arms, and you will see the same number, 408k.
So that would appear to be the number of accountsd that Nexon has.
Assuming that some of those 408K are playing other games and not MS, and that many MS players have multiple accounts, and many of those accounts would be abandoned, there might be only 100k or so actual MS players, maybe even less.

The only thing is that you have to make an account on the forums and link it to your actual account, so that's not an entirely accurate way of determining the real number of accounts.