If I may come in on this, despite posting rarely more recently....
I have got the impression that some forums seem unofficially to lean towards certain sub-areas of kinkdom - quite unofficially, as this is never stated outright. For instance, I have the feeling that there is less interest on this forum in my own main area of interest (being locked in various small containers) now than there used to be - but, on the other hand, interest in cross-dressing and chastity seems to have massively increased (I have zero interest in these particular topics). On the other hand, interest in my own areas now seems to be a lot stronger on FetLife than here, and there are multiple groups there now about bags, cages, boxes, and the like. (That is one reason I rarely post here now, whereas I used to be very frequent 10 years or so ago - apart from generally far lower activity by me now on all forums, on any topics, kinky or vanilla.)
I admit I have not examined this in any detailed way, and it is just an overall impression I have gleaned, and may be unfounded, or based on limited and unrepresentative information.
So is this in fact the case? More to the point, if people have certain particular interests, are there definitely certain forums where they would be better advised to get involved, and others where maybe they may not find so many sharing their interest?
If this is so, it would be very interesting to see a list of various forums, along with information about the particular interests that are stronger in each one. That would be really useful information for relative newcomers not only looking for a forum, but one that will be more interested in what particular things they have to contribute.