Two good intros to Mastodon:
I have the facts to back it up
Look for yourself. Trust me, there's no "big networks" in hiding besides Freenode. And that's 80k users. Mastodon has about 130k as of this writing.
An administrative announcement.
While some folks can be a bit screamy in their virtue-signalling, I am increasingly impressed with the number of people I see on Mastodon/Ostatus having open-minded, non-confrontational, logical, and even courteous-if-not-friendly discussions from divergent places on various social and political spectrums.
That's just plain fucking inspiring.
I wrote an article about some of the protocols underneath StatusNet, and I plan to write more in the near future.
This is my blog post on Mastodon: http://2ality.com/2017/04/mastodon.html
One of the first Mastodon instances, there is no specific topic we're into, just enjoy your time!