This is something you end up discussing every so often. But I think its not about the fact they have ugly interfaces or that they are successful inspite of the ugly interface. I think its about the fact that they were a certain way then they gain critical mass of users, who got used to using these sites in a particular way, knew what to find where and the familiarity with the interface - however it was - grew stronger and stronger. When this happens, if you decide to suddenly change the interface to make it better/sexier/etc, you might be shooting yourself in the foot. I think its important to understand this aspect when you consider such case studies.