I think this has been helped by the general lack of innovation in the email space. From pretty basic mail, we ended up with a few (very surprisingly few) email clients and very little advancement on the original theme outside of html formatting and huge inboxes.
Lots of people dump on Microsoft, but one of the huge upsides of exchange is the tight integration of mail and calendar. From a conversation you can immediately schedule actions. Invites are even sent out over SMTP if I'm not mistaken. Getting a calendar to integrate well with gmail was one of the major accomplishments of web-based email, yet it seems like repeating this anywhere else is an accomplishment comparable to discovering cold fusion.
There's also been pitifully little work done in improving the experience of managing email and calendar servers. Managing spam is still a tremendous problem and all this adds up to most places, if they aren't using Exchange, just buying corporate Outlook.com or gmail accounts for their employees.
The problem of course is that for any serious advancement to really work, everybody (both client and server) have to move to support the advancement.
But one lesson to be learned from Facebook and G+ is that email can be replaced by an easier to use and friendlier system. There's a possibility of disruption, but it's obviously not in anybody's particular interest to keep reinventing email+otherstuff in this kind of highly centralized way. If Facebook goes down, there goes a huge chunk of the global communication system. At least with email I can be pretty sure my message is going to arrive at the destination at some point.
Another lesson to be learned is that social networks like Facebook are actually just a combination and integration of two (or three) common things that used to be all over the web: a personal website and email. You get a profile (which does a good enough approximation of the personal homepages of the web 1.0 days but actually a bit more like ) and people can message you (and more recently IM you). Basically a global presence you don't have to put much effort into to manage and a way to contact you. More importantly Facebook offers you various levels of control over who can see your presence and who can message you. Spam is almost unknown in Facebook's version of email.
So when I see distributed social network efforts like Diaspora, and all this talk of authentication and protocols and whatnot I wonder why we're not really using and extending the distributed infrastructure we already have. Even if we improve it in some way that makes it no longer work with the old email network, it won't be the first time a better internet service replaced a previous one (WWW replaced gopher for example) -- there's no reason two competing distributed messaging services can't run in parallel.