Caroline,
I have not gotten any spam or pop-up ads, but I too have filters. One of my friends told me she has clicked on this link every day now for about 4 years. If she was getting spam or ads, I’m sure she wouldn’t be going there.
What I know:
At The Animal Rescue Site, sponsors become involved with this site as a form of advertising and public relations and thus are willing to pay for their messages to be viewed by consumers. They pay CharityUSA.com, the parent entity of the site, on a per-click basis; CharityUSA.com directs a percentage of the total ad revenue collected towards animals shelters and sanctuaries and keeps the remaining funds. The ads appear on the site, not as pop-ups. (The Animal Rescue Site is not a non-profit entity, so it shouldn’t be confused with a charity even though it does direct a significant portion of its revenues to those in need. It exists to make a profit, and that it’s still around proves it’s succeeding at it.)
CharityUSA also allows visitors to initiate donations to several other causes via The Rainforest Site, The Breast Cancer Site, and The Child Health Site.
Hope that helps.