If I had to guess I'd say that both MySpace and Facebook were completely ad hoc, but the difference was probably that Facebook kept their technical debt down so that they never accumulated an area of code that was too hairy to refactor or replace. MySpace on the other hand probably got complacent in their success and didn't worry too much as their architecture calcified into an unmaintainable mess. By the time they saw Facebook nipping at their heels their code base was past the point of no return, and the company culture made it impossible to hire the level of technical talent they would have needed to right the ship.
Of course I'm completely talking out my ass here, but that's the impression given by the respective product histories.
Maybe just me. I don't know much about the history of the project, except Zuck started it in college.
There's an old saying that you need to build three apps on your framework. That first app shows that your framework works for something. The second app shows that its more general than just the first app. The third app shows that you weren't lucky.
Curious, how was Facebook designed? Are there any post-mortem docs on it? I know that MySpace seemed really ad hoc. I wouldn't be surprised if Facebook was more ad hoc than we imagine it to have been