Actually there was someone who posted a research, where it was stated that the majority of the cash comes from the minority of the players and the amount of those that don't spend any money is quite large, but it provides a lively game for those willing to spend (and that's the main reason why this is a successful model). Maybe it was still back on FoH, can't honestly remember.
What I recently read was this article, that explains how these spending folks are rarely impulse buyers, yet more methodical and careful spenders, but heavily supported the game:
Free-to-play whales more rational than assumed | GamesIndustry.biz
And here it's stated that the market of f2p is growing in the US and declining in Europe:
Free-to-play online games market - Market Data and Reports - SuperData Researchwhich is probably also a derivvate of tihs shitty economic situation the ? currency caused to this continent, but that's another story.