I've pretty much always had this problem when making niche social networks. Get 30-50 people sign up, it gets some very mild interaction in the first month (by typically only a few of the people who signed up), and then dies. (Maybe happened in 5-6 different groups over the years people have started.)
I've taken this to mean the individuals in the group just don't really need another social platform. But I do wonder if there is a better way to get small communities like this to engage so it is more spread out.