I used kbin for a while and now mbin for quite some time. I primarily use the desktop website but one thing I noticed from when I tested some Lemmy mobile clients is that I saw a lot more different lemmynsfw communities & consequently threads, like a lot more.
What I don't know is if this is just a case of this particular instance, or if it is just more noticeable because of the content. kbin was even worse and barely showed ANY content from that instance, mbin was significantly more when I switched, which surprised me. But if we compare for example (pulled from the instance's frontpage):
We get a 404 on mbin. You can repeat this for a lot of communities there with the same result. If you search for this or other "invisible" communities in the Magazine search, then they also don't show up there at all:
If this is the case with other instances then I feel there's potentially a lot of active communities & threads that we aren't even aware of, but also potentially more fringe instances & communities.