Wikipedia gives 677,000 as the size of the Diaspora community

https://en.m.wikipedia.org/wiki/Diaspora_(social_network) citing The Federation: https://the-federation.info

That's ... on the order of Usenet ~1990 or so, per personal conversations with Gene Spafford a few years back. Where "OTO" could be 500k - 5m users. Much of Usenet was far smaller.

Microsoft did some studies on Usenet nodes and behavioural patterns in the early 2000s, and got some usage numbers out of that, though I'd have to dig to find them again.