Money 2007 Generate Key How Do Free-content And Free-membership Websites Earn Money?

How do free-content and free-membership websites earn money? - money 2007 generate key

Google acquired YouTube for $ 1.65 as Google has offered to buy Friendster in 2003 to 30 million U.S. dollars, but were repulsed. Yahoo is rumored to have offered to buy up to $ 1 B, Facebook, and Google has attempted to Yahoo for the delivery of $ 2.3 B outbid. This October 2007 Microsoft announced had bought the share from 1.6% of Facebook for 246 million U.S. dollars!

As far as I know, apart from advertising, Web sites earn money through traffic. ISPs pay for these sites based on traffic from these ISPs is achieved with these sites. (The people pay for internet access, Internet service provider will receive a larger share of these funds will be paid ISP websites). Friendster, Facebook and YouTube are millions of people each day attended.

The question now is:
It can actually trafficking to sites of millions of dollars a year? (I think the ads to generate money only)
If so, can the movement, why Wikipedia is always looking for donations? (Millions of people every day at Wikipedia)