This is an ongoing discussion and now Michael Arrington is at it again. This time it is about Friendfeed having taken 5 months for his account to have half as many friends as in twitter, where it has taken 24 months … now read again. ;)
Anyway, he is comparing apples with oranges, sorry Michael. Friendfeed is something totally different than Twitter. The oh so wunderful thing about Twitter is that there are NO conversations, or only short ones. That reduces the load on your own cognitive capabilities immensely. You go to twitter to hear what people are doing and possibly ask a question which is answered by a few of them. But there is not discussion forming around that one message.
Friendfeed is a simple aggregation service, and yes, it has a few nice features but at least with the current interface, there is no way to follow that conversation. I have added a lot more people than I would on twitter to Friendfeed, including you, I think, but that is only because I value your opinion and when I search for something on Friendfeed I want your opinion to be part of the answer. I will not be following live what you are doing.
So these two are totally different things. I believe Twitter can become a new communication medium if they stay true to what they are doing, meaning that they are not making discussions too easy. Friendfeed might replace Google, but not Twitter.