This FAQ is linked in the comments of the OP, but I thought it bears rereading. It's not just conflict of interest, it's also privacy, and the fact that the community just plain doesn't <i>want</i> advertising. In fact, Spanish Wikipedia already forked once, out of concerns that Wikipedia would become too commercial.<p><a href="http://wikimediafoundation.org/wiki/FAQ/en#Why_doesn.27t_Wikipedia_use_ads_for_revenue.3F" rel="nofollow">http://wikimediafoundation.org/wiki/FAQ/en#Why_doesn.27t_Wik...</a><p>It's true that other projects manage to combine collaboration with some traditional revenue sources such as advertising. But, rightly or wrongly, this is the community's choice, and it seems to be how the general public feels about the site too. There's something about a mission to promote knowledge that people want to keep mentally, physically apart from commerce. A friend of mine (non-techie, non-wikipedian) describes Wikipedia as a "sacred space".<p>Finally, and this is my own take on it, I think there's something valuable about having the reader community take ownership in something, rather than just be eyeballs to be packaged.<p>Disclaimer, I work for the WMF, although not on fundraising.