Speaking just of the U.S.:
There's an article from 2009
stating that 46 million people in the US play MMOs. Another article from 2011
puts this number at 39 million active U.S. MMO players.
Thank you for the links. If you talk about other types of social games, the numbers are even higher. People play online with other people... and they are gamers, even if they don't raid for 3 hours a night. The demographics changed A LOT in the last twenty years, as well.