I remember back in '87, that at least the SE Texas Node, Houston and within maybe 100 miles had about 20 workable BBS access numbers, and maybe 500? users per host.
I know there was some overlap, but assume there wasn't that'd be 10,000 users for a Metro area covering about 5M people. Extrapolate that to a 250M population at the time. Our prehistoric internet community would be in the neighborhood 500,000, but I am guessing it probably substantially less than that.