How effective are the ads in the page footer? The ones at the way bottom of the webpage?

I'm just wondering, because I hardly ever (if ever) scroll down far enough to even see them. So much so that I forget that they're there often and am surprised when I do scroll down to see them.

These ads are ALWAYS there, yet the top ones seem to come and go. Wouldn't it be more effective to just always show the top ad and do away with the bottom ad?

Just curious.