Dota 2 has been the most popular game on Steam for a while now, and with the gradual rollout of beta keys the number playing has been steadily climbing - you can see a graph of the number of players on steamgraph[1].

[1]: http://steamgraph.net/index.php?action=graph&jstime=1&appid=...

No, it's been the most popular Steam title for well over a year now. It's been several times more popular than the second most (usually Team Fortress 2) for quite some time as well.

Furthermore, even though they've just announced that they've come out of Beta, almost none of the new players have been invited in yet as they're inviting people via waves.

Once all new players actually get in, and Dota 2 opens up in other countries such as China and Korea, these figures will skyrocket by several magnitudes - probably well overtaking World of Warcraft in number of players.

Partially responsible, sure, but DotA has a very dedicated fanbase, stretching back to the days of the first DotA map in Warcraft 3. Also, it's a game that offers huge replayability, meaning the current number of people playing the game will likely stay stable, if not increase.

Dota2 being the most commonly played game on Steam has been true for several months. During which time it was "Beta", not that it was hard to get your hands on a beta key.

It didn't really just launch though, its been called beta but in reality it has been released for a very long time.