Friendster -> MySpace : Poor Technical Execution -> Good Technical Execution, Less Features -> More Features, Niche/Techie -> Mainstream.

I would have to say that FriendFeed may take over in the Techie crowd, but it's not the mainstream equivalent of Twitter because it is too reliant on existing usage of other techie sites.

Facebook, with a little tweaking, is well positioned to be the mainstream Twitter. It already has a 'Feed', Mobile Updates, IM client. You may argue that the essence of Twitter is more than the sum of those parts, but for the Mainstream, a tweaked Mobile Personal Facebook Messaging would probably make an acceptable Twitter.

Alternately, Plurk has a very 'teen high school girls would love this' feel to it. Very interested to see how it develops.

It's Twoorl (http://twoorl.com), of course :) Twoorl is open source and it's written in Erlang.

Twoorl is just a clone of Twitter, whereas Myspace extended past Friendster's features and made it its own site.

Plurk and Pownce are different enough from Twitter that they're not seen as just clones, that they fall into the competitor's department.

Twoorl started as a Twitter clone, but it doesn't mean it'll stay as such.