http://euw.leagueoflegends.com/news/you ... ssive-game

I knew it was big, but didn't realise it was quite this big. It wouldn't surprise me if this places it as the second most played game atm (the first being, of course, WoW).

Some key numbers, for TLDR purposes:
15mil accounts.
4mil players per month (well, summoners, so could be a fair few smurfs)
1.4mil per day.
0.5mil simultaneous players (at peak).
10 games started per second.

http://www.xfire.com/games/# (Sort by Rank)

According to XFire, LoL has overtaken it as the most played game this week... with 8000 more hours... Which is still 8.5 times that of Team Fortress 2.