I was a big MySpace user and late FB user, and I can tell you that it had nothing to do with [tech] scale. Back then I didn't know about programming (my first lines of code were some CSS in MySpace!) and the real reason is that users were becoming more hostile in a site that was a niche on its own. You had to start filtering out and removing SPAM all the time from your public wall. The fact that people could personalize their site and the community (heavy on music and alternative people) put most of the people away.

You can say that it failed to scale, but I wouldn't say that it failed to scale technically; it failed to scale the community. Then at some point when it was already dying they started to change the layout big time I guess to attract more people, alienating the initial community.