I'll stick my neck out and say Facebook has an awesome UI. (No idea what was used to make it.) When I first joined Facebook some weeks ago, I was floored by how many features there are and how every single one is usability-optimized for me the newcomer, catching my eye exactly when I need it. The attention to detail is amazing. Sure, it's all just text, pics and videos, but... I'd been making online maps for years, have some fairly advanced stuff under my belt, and still felt like a newbie when I saw Facebook.