So just by nature of tolls you wouldn't call many out of area BBSes. If you did it was brief calls for a post or to check mail but not sit around playing a door game or downloading some big file.
That all meant BBSes were very local for most people. This is in addition to the fact only a small fraction of homes had computers, a fraction of those had modems, and a fraction of those actually called BBSes. That aspect also meant the populations were relatively small. A small web forum might have a hundred users spread over a large area while a BBS would have a dozen in the same town.