I recently looked up Wikipedia.com on Alexa and I was really suprised that they had an Alexa rating of "9". This website obviously gets millions of visitors and doesn't seem to place any advertisments or Adsense ads anywhere on their site. If I'm right, this means that this company is losing out on hundreds of millions of dollars in advertising revenue.
Yes they earn money on donations, they are also losing a lot of advertising revenue. But how trustworthy would it be if companies advertising at the site?
As previously mentioned, donations, well they call it "fundraising". The page with more information on is here
Donations. They also get a large volume of their traffic from Google for being listed so good for so many keywords and phrases.