Email tools?

I am checking out an email tool that looks pretty interesting  -- xobni. I have been using Facebook more lately and have been thinking about how email and Facebook are similar types of systems.  There is a social graph, a good way to communication async, and a sense of dealing first with "who" then what.  I pretty much always read the name of the person before I think about the contact/mail message.  Same in Facebook.

Have you used xobni yet?  What is good about it? What is bad?  Do you use facebook too? How are they different?

Here is a link if you want to check it out.