In all the years I was on Stack Exchange, I never really understood the community promotion ads. I can see why SE would put up advertising, since that's their business. It didn't make sense to me why the community would want to advertise something else. That said, is it really fair to use the mechanism SE has provided to promote their competitors (I expect that's how they will see it)? It feels like like exploiting an accidental loophole.

@Olin different sites use ads in different ways. Some promote their own community resources, like a blog or question sandbox. Some promote resources the community considers useful to its own community, like reference libraries and tools. If the community here was founded by the community there, then it's not unreasonable to use the mechanism to promote this community. Whether it's a good idea is a question for the people who are/were active there.