For those wondering, at the time of writing:
- Discord has 174 users online in #general, with 989 registered (according to @spdegabrielle)
- Slack has 1607 users registered in #general, not all of whom are online/active (difficult to get)
- IRC has 130 users online, with many more registered
Obivously, "currently online" numbers will fluctuate a lot with timezones, and "total users" is totally different than online/active.
The Racket community is quite splintered it seems. 