Facebook has opened up its platform for extensible development by the community, so expect custom widgets on par with MySpace, but I believe, with far, far more reach.
We are seeing companies prefer open source solutions (Drupal, Joomla) over proprietary crap. Why? Rather have 300 000 developers behind your product than 5. Another thing is that prop software is so difficult to port or extend, you have to imagineer yourself into the original guys mind, and really: do you want to?
this is a brilliant move by facebook. Software guys may code widgets for FireFox and Yahoo!, Drupal and even PHP to up their geek credits, but what if you can code on facebook and impress ALL your friends? For the college crowd that make up the numbers at FB, these guys will be up all night and facebook is going to be the next public space (ww2?)