Well, for now I’d call them house ads, because as far as I can tell the ad block below the follower data on your Twitter profile just links to Twitter search results. Which makes sense really, since the real time search that Twitter offers might be the best way to monetize the site.
Either way, if Twitter can sell these kind of ads, I’m absolutely OK with it. First of all, I rarely visit the web interface, and second, it’s not all that obtrusive in the first place, is it? It would be cool if I could
get Twitter to match the ad with people who might want to follow me, so that I could use it to build a larger follower count. At least, that would be an interesting use for brands and companies.