Wikipedia is in my opinion unluckily a complex example because the donations are actually for Wikimedia.
In my case I donated only once to them and I understood only later that (apparently?) most of the $ isn't being used for Wikipedia, therefore as I'm not interested in their other projects I never donated anything anymore to them - they can make that banner as big&flashy as they want but the fact remains that I don't want my $ used for non-Wikipedia-related activities.
