Or, FriendFeed Is In Danger Of Becoming The Coolest App Michael Arrington Doesn't Use.
I'm a friendfeed member, but I dont find it all that useful. And yet I do find it well designed, so it isnt a usability issue. I use facebook, and I use twitter. For me, these are separate bases of people. I like to send more updates to twitter than I do facebook, and I dont want to link to pictures etc via twitter, etc.
In addition, while I use many of the other connection services friendfeed offers, not in any real way that makes sense to connect them together.
In the end, I think friendfeed is useful for those who use a large number of the services and want to get it all together -- but trying to explain friendfeed to people who are just starting to use twitter is very difficult, and its going to be a long time before they see any usefulness out of it. Twitter and Facebook do not have this problem, at all.
If I was a part of a large number of services and I didn't want to check them all the time, I suppose I might use FF. Facebook and Twitter, though, are too distinct to be paired the way FF offers, however.