While we are on the topic, I realized (reading Brad's new post) that I have been neglectin to mention a very significant current owner of the social graph:  e-mail.

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.