Caveat: email as we know it developed in frontier times when things could grow organically and/or by general consensus rather than being driven by commercial needs.
The frontier days for this sort of communication are over for the most part. Anything that shows signs of being significant will get jumped upon by commercial entities who will try to direct it in a manner that helps their larger goals (and if we are more cynical, expressly in a manner inconvenient to their competitor's goals) potentially (in fact most likely) at the expense of the overall scheme (despite publicly doing it to "help the users/community/public".
Anything new now has forces acting to shape it (for better or, often, for worse) that email never had to contend with until it was so widespread no one entity or collection of entities could wreck it for everyone (despite the best efforts of some such entities!).
well, the new frontier is "dark mail" or more generally "dark net" where interoperability again is the key [as the only way to grow. It is like natural evolution - any non-interoperability efforts will just not grow, and only an interoperability based effort will produce the critical mass]. It is organically decentralized and by its nature a user is the owner of its data even though actual [meaningless to 3rd party] bits may be stored by one or spread around multiple distributed providers ("dark money" style as if you BT wallet is encrypted and replicated though the BT network itself). Content mining and "user is the product", and thus FB and Google as we know it, would be for some time problematic at best and it will be some time before segmentation era of the new net dawns and new Serge/Larry/Zuckenberg appear :)
But it isn't an empty frontier that the thinkers/developers are entering alone. The existing problems both commercial and legal (and not-quite-so-legal-but-state-funded-so-good-luck-fighting-that-one) will be so close behind to stop/exploit/both as to make the exploration very inconvenient. I'm not saying it can't happen, but it won't happen organically over a period of time like email did.
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.
Some people are worried about Facebook taking over the internet? First of all, I'm worried about the web taking over the internet. And becoming more and more centralized in the process.
Alas, any truly distributed scheme will face the lack of symmetric bandwidth. Which is why MegaUpload ever existed in the first place: if we could upload as fast as we download, distributed networks such as Bittorent would be too damn efficient. (On average, we're ultimately limited by the smallest of the two. Upload minus download is exactly zero over a closed peer to peer network.)
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.
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.
The frontier days for this sort of communication are over for the most part. Anything that shows signs of being significant will get jumped upon by commercial entities who will try to direct it in a manner that helps their larger goals (and if we are more cynical, expressly in a manner inconvenient to their competitor's goals) potentially (in fact most likely) at the expense of the overall scheme (despite publicly doing it to "help the users/community/public".
Anything new now has forces acting to shape it (for better or, often, for worse) that email never had to contend with until it was so widespread no one entity or collection of entities could wreck it for everyone (despite the best efforts of some such entities!).