Can't really figure out how this is possible, even applications that you install shouldn't be allowed to send chat messages on your behalf. Perhaps browser malware?
Someone referred this song to me in Facebook. Pretty funny =D
Speaking of Facebook, it's pretty interesting in itself, because of the way "developers" (aka anyone) can extend its functionalities by writing apps for it. This way people actually make it interesting by creating content and apps, rather than just Facebook creating all the applications themselves.
The question to be asked though: will this really last?
I suspect not actually. It will last longer than Friendster and MySpace perhaps, but the human desire for more and more "new" things will catch up (sooner or later). Just a thought.