I fully agree with Brad. Along with a large swath of the social graph, e-mail also packs a ton of context with it. Years ago, while building SelectMinds, we came across a company called Tacit (since acquired by Oracle) that attempted to abstract some of this context and social data out of enterprise e-mail. I have no idea how the product faired under Oracle. I can't imagine they are doing anything interesting with it, because otherwise, I think I'd have heard about it.
E-mail's also got a ton of attention. I pretty much live in my Gmail account. It's the homepage on my browser. It knows more about me that any other property.
I am not very familiar with Gist (of the two companies that Brad mentions) but Xobni got a try for me. However, after a year or so using Xobni in my Outlook (over which I still access a few IMAP e-mail accounts), I did not get much value out of it, so it did not survive my hard disk swap a few months ago.
I am looking forward to innovation that will extract some social data out of e-mail.