Hmmm..speaking as a moderator for a top 100 big board, we have a huge problem with people joining just to promote their own sites. If you do it on my board, you'll get an instant ban and a deletion. We do allow people, even newbies, to put a discrete link in their signature. That means they at least have to participate in the boards to get seen, but if they post a bunch of nonsense for that reason, they are gone. So, if nobody has mentioned it before, please be considerate of the rules of the board you are joining. Participate, don't spam.
That said, there are no boards on my specific topic, and I'm having a difficult time finding one on the broadest topic. I have at times put a sig link in with the board I moderate, which gets me huge results. I suppose I can try it again although as a moderator, I'm not always popular with the regulars, and do fear some backlash to my blog which is why I took it out.
Since my topic is about being inside a public school, I was looking for general school employee forums, but found none. I don't belong on teacher forums, although I can't find those either. The only other group who would be interested in my topic, possibly, are parents of young teens so maybe I should start at some mom boards.