In order to keep a continous conversation going across blogs, twitter, socialnetworks, etc It would be great to have the equivalent of the @ reply on twitter available across all sites. That way you could instantly reference via a link any persons or companies mentioned, on the fly. Some examples:
Right now, for us twitter users, Facebook is a bit static when it comes to the status updates. Its really designed to be more of a one-way broadcast out to your friends of what you are doing, rather than a dynamic conversation. My take is that the wall discussions are really where Facebook wants you to have your discussions, and they include that functionality in the mobile app. Either way, when I'm checking status updates on the mobile app I find myself wanting to reply right away just like on twitter, and I find myself wanting to reference other facebook users within my status update. This would make the status much more dynamic and social!
Blog commenting engines like Disqus already do a lot to aggregate conversations across the web into one place, but I find myself wanting to be able to reference users from within the comments. Maybe its because USV invested in both Disqus and Twitter, but it just feels like you should be able to do it, that way conversations can carry from twitter, where someone posted a link to a blog post, right on through to the comments.
The big question then would be, how do you do it? Maybe a facebook connect like API that lets people use "@" + username from twitter across all sites? Or would you want to be able to @ your facebook friends, creating links back to their profiles?