We have seen several different types of ads in the sidebar of the sites. We also have Area 51 ads:
These ads are not shown in the sidebars.
What would you say of adding these to the sidebars as well?
As we're getting some crap on Area 51 as well, perhaps there should be some follower amount threshold before the ad to appear in a sidebar, but that's a detail.
Also, as Sha Wiz Dow Ard says in the comments, the engine can track the user's tags to see in what he's interested. In addition to that, the engine could show more Technology proposals on Technology websites, and the same goes for other categories.