Cliff Gerrish writes on his Echovar blog:
The consumption strategy that makes the instant messaging model of Twitter work is to follow a core group and then track keywords of interest. Tracking keywords adds people you don’t follow into your stream and provides a proper level of noise and negative feedback into the information ecosystem. ... It’s tracking that makes a decentralized Twitter nearly impossible. ... This feature radically changes the shape of the social graph underlying the information stream. Since you don’t know who might use a tag you’re tracking, the regular RSS style contract around publication and subscription doesn’t work.
Gerrish's claim that tracking makes it impossible to decentralize Twitter seems to have convinced Steve Gilmore of Techcrunch who writes: "Decentralizing Twitter is unnecessary, if not impractical." Fortunately, both Cliff and Steve are wrong.
Distributed tracking of Twitter-like streams is easily accomplished using what are now well-known systems for distributed publish/subscribe. Certainly, it is easier to implement tracking if you have everything going through a single choke-point in the network, but it isn't necessary. In fact, as long ago as the 80's we had USENET based systems that handled the distributed fan-out of messages (news posts) that were then "matched" against user's local subscriptions (Yes, matching was normally trivial "topic-based" matching, however, "content-based" matching systems were often deployed locally). What we could do in the 80's we can do today -- but do it better. After all, we've learned a great deal since then.