"The evidence that Google+ wasn't ready to be a Facebook killer is that it is invitation only."

Google knows the best way to release a product - especially one in unfamiliar territory like the social space - is not to flip a giant switch and hope that when the whole world starts using it, nothing goes wrong. For one, the scalability issues when you're Google are enormous: they know they'll have tens of millions of 7-day users the first 7 days. You don't want to screw that up. That doesn't even touch on product design.