I've been digging up numbers over the past few years. In 1980 there were 2 million computers in the US, doubling every 2 years. By 2000, there were 168 million computers, only 6 doublings rather than the 10 the 1980 estimate would have provided. That suggests about 16 million users as of 1990, possibly 24-32 million by 1992.
As of 1995, total worldwide Internet usage (then largely in the US, though also Europe) was 16 millions. As of 2019 it's 4.5 billions.
https://www.internetworldstats.com/emarketing.htm
(From an earlier HN comment: https://news.ycombinator.com/item?id=21783812)
Brian Reid's Usenet Usenet usage reports as of 1988 reported about 140k active Usenet users, from a population-with-access of about 880,000.
Usenet was small.
(Some of these stats also cited in in one of the links (which I wrote) from TFA.)