I am basing my numbers on this link from Magnus Carlsen's interview on the BBC. (despite the fact that the presenter knows nothing about chess. He is white and is talking about how he is playing the sicillian defense)
http://www.bbc.co.uk/news/uk-21835323
Anyway, the expert here is saying that UGOV did a poll for them and discovered that about 70% of people have played chess at one time or another.
Most people don't play? They reckon in the UK that 70% of children play and 30% of adults. There are chess clubs in almost every primary school.
Those pushing to add chess to the Olympics estimate that there are 605 million players worldwide (less than 10% of the world population), but this doesn't tell us how much these players know about chess or how often they play. The second link below asks some of the tough questions about this estimation process, and simply doubts (as I do) that the numbers are anywhere near that high.
For one, I'd expect the biggest chess websites to have many more players as members. Chess.com has just over 1% of the estimated number of worldwide players, but Chess.com's 6.8 million registered users likely includes a mere fraction of active players (and likely counts a player who has created dozens of accounts as dozens of players--my guess).
http://en.chessbase.com/home/TabId/211/PostId/4008392
http://www.kenilworthchessclub.org/kenilworthian/2007/02/how-many-chessplayers.html