DotA 2 is officially the most played game on Steam as of now.
Skyrim previous held the record for the most concurrent players back in November 2011 at 287, 411 players. However, this number decreased rather quickly throughout the years since its release. Since November 2011, DotA 2 has been racking up more and more players to the point that it has now peaked at 297, 010 beating Skyrim’s record.
DotA 2 is still in its beta phase so, from here, it can only go up.