After much resistance, I finally joined Facebook... because I wanted to see what their JavaScript looked like. I admit, I could have done this without signing up properly, but just as I was contemplating signing up, someone sent me an email with links to baby pictures and I finally caved.
Anyhow, I haven't seen anything too spectacularly interesting in their code yet, but this snippet did make me laugh:
function URI(uri){if(uri===window){Util.error('what the hell are you doing');return;}
Classy, eh?