I had a play with medium the other day. It's an interesting bit of software - it installs a sidepanel on your browser and then creates a visual radar of sites similar to the one you are visiting. You can see which other medium users are at these sites. You chat with them by putting out a general call on the bulletin board. I found it a bit disconcerting though, having these people watch the sites you visit. You can make yourself anonymous, but even so the thought of being observed feels rather intrusive, and so I turned it off and it has remained that way since. I also found the means of communicating didn't really lend itself to decent conversation with people posting rather banal comments.
As I said in an earlier post on Gabbly, I think there is something in these tools for e-learning as they can promote informal discussion around content. I think medium would need to work on the group allocation though - I can see it being a good tool for an agreed set of users to collaborate on a specific task, for instance a tutor group could perform online research using it. There is a friends option, so maybe this could perform that function (as usual I didn't have any friends so couldn't experiment) but I think you'd want to allow simple group allocation and lose the general waffle.
The scrobbling of sites you visit to create the web radar is a good idea also, although I don't want others observing what I do (not that I do anything unsavoury, unless one counts going to some Spurs discussion sites).