Is that true?
Peak IRC users appears to be 10 million (2013) - https://en.wikipedia.org/wiki/Usenet#Decline, Citation needed.
Pretending that all IRC users were in the US... That's ~3% of the population.
Usenet is arguably going strong (daily volume has exploded but isn't a useful metric here though. Can't find active users from cursory look).
~70% of US population are 'active' users of Facebook. This counts people that have only used it once in the last 30 days. And it's not a dichotomy - I don't think people have an issue stepping outside of facebook, as long as there are no barriers to entry.
Just playing devil's advocate, I don't disagree with you in general. I appreciate that my stats are lazy and blunt, but it shows how relatively minuscule the usage of IRC & Usenet were 2 decades ago, and that while direct messaging might be in a more 'walled garden' state, traffic to self-federated content is generally still as attainable. Perhaps 'eternal september' has swung so far to the other side that the old non-internet users of the 90s have funnelled through AOL and out the other side to Facebook (and notably, it's easier to grab traffic from FB users than non-internet users), while the core tech demographic has remained 'in the middle'.