This answer rightly points out that part of what makes SE work is the multi-site aspect: experienced users on one SE site will naturally start looking at others too, and instead of 130 isolated communities you end up with more cross-fertilization.
When this site launched, as Moderators, we tried to reach moderators of other SE sites. But that's too narrow, and we're now Community Building instead. Where else on the SE network should we be looking for users who might also be interested in this site? Which sites' topics are somehow related to our topics?
If we can identify relevant sites, we might be able to propose some cross-site ads.