G+ will be popular with the tech set (and probably their freinds), but I can't see moms, dads, grand parents really switching to it. Nor can I see it becoming as popular until there are games, etc. For better or worse, the vast proportion of FBs users will not move over.
Take a look around, how many people still have yahoo, aol, and MSN email addresses? Yahoo still is a very popular destination for people (groups, etc).
The concept of social networking was still being defined. facebook is social networking in most people's minds. The concept has been solidified. Google's recent offering does nothing to reinvent the concept.
Also, social networking was still something mostly young people were into. The network-effects now are much greater in magnitude than they were then.
The biggest factor was that myspace was perceived as a "ghetto" (I had multiple people describe it to me as such). People didn't join facebook because it was more compelling, they left myspace because it had degenerated to a ghetto and facebook was the most viable option.
The social networking battle is facebook's to lose, not google's to win.
"The concept of social networking" is very much still being defined today. The objection to having parents/bosses/other "uncool" people on Facebook is but one example. The entire idea is something like 10 years old and incredibly far from maturity and being set in stone.
Indeed, Inertia shouldn't be an excuse for Facebook to not improve their track record. Atleast now that they have some competition, they might actually get better.
Take a look around, how many people still have yahoo, aol, and MSN email addresses? Yahoo still is a very popular destination for people (groups, etc).