Want to advertise on Facebook? Well, doing so is subject to approval -- nothing unusual there. But just who makes the cut and who is likely to be expelled may be surprising.

The story first broke on TechCrunch, when 3Jam CEO Andy Jagoe emailed the technology blog to report that his company name had been blacklisted from Facebook.

That information prompted blogger Erick Schonfeld to launch his own mini-investigation.

Competing social networks like MySpace, Friendster, Hi5 and Orkut are all forbidden from running ads on Facebook. While the policy would seem logical -- a social network wouldn't want to promote a rival -- the execution is more than a little peculiar. After all, Bebo, AOL, Yahoo and OpenSocial don't raise any alarms at Facebook's automated advertising page. Ditto for Microsoft, though that's no surprise given Microsoft's stake in Facebook.

As for the ads that Facebook actually accepts, advertisers may be happy to learn that the social network is quietly rolling out a feature that lets users give feedback.

The new feature allows users to toggle a positive or negative feedback button adjacent to the ad. When an ad is marked negative, users can select from a list of reasons why the ad failed to engage them. The reasons include: "misleading," "irrelevant" and "uninteresting." A different set of answers is served up when users give an ad a positive review.

While Facebook isn't actively promoting the feature yet, the company does seem to be taking a more deliberate and cautious approach to advertising after the launch of Beacon last year nearly caused a meltdown for the company. Recently, Facebook has quietly been touting the success of branded gifts, though it will be interesting to see how users rate the new ad units.