And I finally have an alternative to Facebook: extensible, awesome, and Open Source. [EDIT] ELGG is written in PHP and is completely open source. And it comes with some excellent documentation all well written and covering a wide range of topics to get everyone the help they need - whether you're just trying to install ELGG or write plugins for it, or contribute code for the next version.
Here's a comparison with Facebook:
sadly, there are limits to how good a face I can present.
Plugins. Themes. Customization.
Everything you've wanted to ask Facebook for, but were afraid to.
Update: There are even themes to mimic the whole facebook look. I have not been able to customize the instance as much as I want to because I'm having trouble finding my way through the documentation. Maybe today just isn't the right day to wend my way through pages and pages of documentation. There are some kinks to be worked out. Clicking on a link to list pages in the Installation category took me to an error page stating that I did not have the required privileges to edit the page.
But there are various examples of ELGG instances that have been customized to look nothing like the default install I currently have up.
Also, I doubt anybody who still uses Facebook daily without any concern for their IP policy will really find any reason to move to an ELGG instance. That includes most of my friends. Maybe making the interface more click-whiz-popup-ish will change that. Right now, it's very much new-page-loading based. But it's a great system nonetheless. I am amazed by the fact that this is all very very open source (I haven't been able to find a mention of the license it's been released under, still, maybe I actually need to look into the code, UPDATE: Wikipedia says it's GPL v2)
Maybe I've been spoilt by Anil Dash and his pushbutton web post. Maybe I'm just hoping for something better than Facebook right now. Anything would do.
I'm even trying to set up a Wordpress MU instance now. And yes, it's for BuddyPress. Right now, I can't seem to get past the wp-config.php stage. Will update when I do.