Even if they chose to be driven by donations, I'd have imagined one the biggest website on the internet has better, more effective ways of raising money.
Is it really that bad for non-profits on the internet?
How many of them are good for users, though? Sure, you can slap some Google AdWords on there, but there's a reputational and privacy cost. NPR-style sponsorships? Maybe, but I cringe when NPR does a story on Facebook that's bookended by "Facebook is a sponsor of NPR" a little.
I'm a fan of the current model, in that it makes a conflict of interest quite unlikely.
With advertisers you build your product to sap as much attention as possible.
With corporate sponsors you start to let them moderate your product.
With annoying users for money you at least want your users to like you instead of making them a product of some kind or another.
That way everyone will learn its value to them and they could collect some $$$