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Your argument was that people aren't using servers that run FLOSS federated software is because they're not aware of them. My argument is that the people who aren't aware are also people who wouldn't use those servers even if they knew.

The people who run their own mail servers are massive nerds, the people who are on Mastodon are also massive nerds. This status quo won't change just because a megacorp adopts ActivityPub (in fact, you eloquently bring to attention how everyone uses Gmail or Hotmail, and basically no individual runs their own mail server*). So if that's your argument, the original commenter's point still stands.

*Moreover I've heard that because of this monopoly on email, sending email from a mail server you run to the big providers without it immediately getting sent to spam, if delivered at all, is basically impossible.