I don't think email is a good replacement for Facebook. Rather what we need is open (email protocol like) social networks. The problem with Facebook is that its a monopoly, and closed. What if it were open, such that I can send a friend request from G+ to Facebook. Just like I can easily send an email from yahoo email id to gmail. This openness, if incorporated, can solve the monopoly problem as well. Plus we don't h…
That is what diaspora tried to do. It has not gone well.
Two of them were:
* Their federation protocol was (still is?) very poorly documented. This made alternate implementations difficult.
* A user cannot move his data from one Diaspora server to another. (This might have changed in the last six to twelve months, but I doubt it.) This sort of moots the benefit of decentralization; what good is it if I can pick from any one of a sea of hosts, but must stick with that host for the rest of forever, even if they turn evil?