The official Dota 2 blog is home to a counter tracking the total number of unique players the game has seen in the last month. As noticed by Reddit user dagla, that counter shows Dota 2's players have eclipsed 7.86 million in the last month--a figure which would put it slightly ahead of the 7.8 million subscribers World of Warcraft had when Activision last reported numbers in February.
There is, of course, an important distinction to be made: World of Warcraft players are subscribers, all of which fork over $15 for access to the game every month (unless they elect for a longer-term subscription, in which case they get a discount).
Dota 2, by contrast, is a free-to-play game, and a very unaggressive one in many ways. Unlike competitor League of Legends, it allows players access to all of its characters for free. It instead makes money off sales of cosmetic items, new announcer voice packs, and other things that don't have a direct effect on gameplay.
Generally speaking, only a small percentage of a free-to-play game's players ever spend money--a recent report suggested that number was 2.2%. While it's possible the paying players in Dota 2 spend more than $15 per month, we have no precise way of comparing how much money Blizzard and Valve are making off of these two games.
Also of note, while Dota 2's 7.86 million players is an impressive number, it pales in comparison to League of Legends' player count. Earlier this year, developer Riot Games announced the game was seeing upwards of 27 million players every day, and an astounding total of 67 million players per month.
Valve released its first-ever documentary film, Free to Play, last month. It chronicles the stories of three professional Dota 2 players as they prepared to play in the first-ever Dota 2 International Tournament. More recently, it launched the Spring Cleaning update for Dota 2, introducing a wide range of changes and bug fixes.