Right, and one of the reasons this doesn't happen is that Facebook doesn't get any benefit out of donating all that money, other than the (potential) control they could get by threatening to stop funding Wikipedia.
Businesses have an incentive to buy ads, and users have less incentive to donate to an ad-supported site, which means that the funding would be skewed drastically in favor of advertisers. With donations, yes, you can have large donors using their clout, but you don't automatically skew the pool of money towards a few particular interests.
Ads also have problems in that they are juxtaposed with the content of the site. That can create an implicit sense of endorsement of the ad by Wikipedia, and vice versa. People manipulate content providers all the time, by mentioning to advertisers that their ads are juxtaposed with some objectionable content, and even threatening to boycott those advertisers if they don't remove their ads. By only accepting donations, you are supported by people who are much more likely to be dedicated to the free exchange of information that is your mission, and you aren't putting their name up next to content that they might object to.
Businesses have an incentive to buy ads, and users have less incentive to donate to an ad-supported site, which means that the funding would be skewed drastically in favor of advertisers. With donations, yes, you can have large donors using their clout, but you don't automatically skew the pool of money towards a few particular interests.
Ads also have problems in that they are juxtaposed with the content of the site. That can create an implicit sense of endorsement of the ad by Wikipedia, and vice versa. People manipulate content providers all the time, by mentioning to advertisers that their ads are juxtaposed with some objectionable content, and even threatening to boycott those advertisers if they don't remove their ads. By only accepting donations, you are supported by people who are much more likely to be dedicated to the free exchange of information that is your mission, and you aren't putting their name up next to content that they might object to.