Personally, I might use the term "bubble" rather than "echo chamber," because so much of Mastodon's design focuses on giving server administrators tools to block things they don't want to see, and "bubble" emphasizes the barrier-forming aspect.

"Echo chamber" IMO put more emphasis on the "other people will tell me I'm correct and we'll all convince ourselves there are no significant divergent viewpoints" and I don't really think that's true. It's certainly not dominant.

Hashtag nuance.