There are some communities that do overlap significantly. Personally I find that poly, kinky, gamers and pagan circles are very interwoven. People who are fluid in sexuality and/or gender are also often commonly woven in too. I also find that these identities - not solely a community - are often overlapping as well. Of course, one can identify with any of these without any of the others. It might be that people in these communities may be more 'organized' to use River's term in that they go to events, classes, participate in forums, parties, rituals, so they appear to be interwoven more than their actual numbers. The numbers of people experimenting with kink is way more than the people who are active in any given kink community. Ditto with poly or pagan. (I don't think gamers have the same dynamic here but could be wrong.)
So, lt:dr, seems to be much overlap but who knows for sure?