There is a certain sense of responsibility and ownership in donating that, I’m afraid, switching to a anonymous advert based model may loose.
Not that I own Wikipedia if I only donate a $100 but I feel more inclined to support it and, yes, advertise about it than I would if such a relationship disappeared.
This relation, in my opinion, is similar to the one those who contribute build in time with the site. This is in essence the type of relationship the Wiki concept is based upon.
Besides, isn’t there a different business model than advertisement?
Originally posted as a comment on /Message using Disqus.