I think that Facebook killed MySpace for one simple reason - tricking people into using their real name made it easy to find real life friends (and classmates, etc). IMO, this is what put it head and shoulders above all the other social networks. Nobody could copy it, because their existing users (the early adopters) had silly names like "wisty".

Existing users already have old habits, and existing data that they don't want to retype. Users aren't code - you can't just refactor them into something new, clean, and shiny.

Google+ users will build their networks afresh, focusing on privacy and groups. They will just maintain two accounts, and use Google+ when they want some level of privacy, and Facebook when they need to announce their wedding to the late adopters.