It's not identical to facebook or IRC either, that's my point as well. It's a new sort of thing that requires careful thought to properly understand, applying inaccurate labels doesn't help.

It's clear what twitter is: it's a website, it's a SMS service for phones, messaging, service, etc.

There is plenty of vertical development that can be done by third parties, no need to focus on creating your own client.

In programming, twitter is like the IE mini-browser control you can drop into a window and create your own 'custom' browser. Clients like that aren't needed. Instead use the data in a new way, or provide a new data stream by tracking something in real life.