An easy explanation would be that that while Twitter has technical issues, more important is that it has a network of users. The strength of that network cuts off at the pass anyone who challenges them. I don't buy that argument. Networks, like swarms, move frequently. Friendster to MySpace to Facebook. No network effect prevented that migration.
In my mind, to entice people to switch networks, you need to offer them something new. Copying the existing market-leader won't do it. You basically need that one killer feature which defines the product and gives people a REASON to move.
In MySpace's situation, I actually think their feature was (a) it was an LA-based company; and (b) they appealed to the music scene. Those aren't typical product features, but in this case they were. Everyone in the bay area was on Friendster - there's no way a bay area social network that appealed to the same crowd was going to win. MySpace went after a different crowd in a different way and the swarm migrated.
Emily commented this on my last post on Twitter:
The complication added by the axillary features like you find on pownce is just annoying for someone that likes elegant, clean, simplicity.
Judging from their traffic, she's 100% right. The feature set of the company hasn't given anyone a reason to move. - at least for the bay area, early adopter crowd that Emily is part of.
Who knows, maybe the Twitter-killer feature is geographic, like MySpace, and the ultimate Twitter killer needs to come from someplace like LA or New York. Or maybe there is no Twitter-killer feature and the network will rule (like with eBay). But for some reason I doubt it.