[icebraining]

Less and less of a counter example. If you leave out work mailboxes, most people these days don't send their email any more: they ask Google, Microsoft, or their ISP to do it from them, from their web browser.

Federated is not the same as peer-to-peer. While there are big providers, email is still open to anybody with a non-dynamic IP address, and the fact that almost everyone uses third-party servers doesn't change that.

[loup-vaillant]

My point is, there are fewer and fewer email providers, and they are getting bigger and bigger. If for some reason 95% of the population had a gmail account, Google could be tempted to shut down all SMTP communications. Technically that would remove them from the federation. Practically, that would remove everyone else from the federation (they are on the bigger half on the network, after all).

Another, more realistic possibility is that a consortium arises to fight spam, and the result is that email that doesn't come from big players or registered entities will simply be ignored, on the grounds that most mail from little domains would be spam (which by the way is probably true). The next step will be about refraining themselves to send email that look like spam, never mind the freedom of speech issues that will ensue.

And voilà, we have a world without spam. Too bad I can't send email to most of my friends any more (I control my own domain name).

Now Facebook is even worse. It already took over. You could try and build a federation, but unless you can force Facebook to communicate with it, it will still be a mere competitor, which starts at the wrong side of the network effect. Just like any Facebook competitor, actually.