Advertising on SO. Sure, you can get highly targeted advertising that could help solve peoples problems. Converting Oracle to MySQL could have Oracle promoting tools to do just that on that very page. However, all those impressions are essentially people coming to the site, looking for an answer, and leaving. If the answers aren't there, they go back to Google and look for another answer.

Advertising on something like SO is almost like advertising your latest application on the Pirate Bay. I just don't see the value.