You get one shot where each user will try your product, and decide to stick with it a while, or write it off.
If I get a Google+ account, if my friends aren't on it after a few days, then I think its crap because its got no content, and I go back to facebook. When my friend of now tries Google+, I'm not there to talk to; and the launch fails.
Its great to seed your product launch with influential early adopters. But these early adopters don't form a sufficiently dense social network to make the product worthwhile, on their own.
The adoption of a social product spreads like an epidemic. The frontier is either pushing out through the network, or its dying out.
I believe they should have stayed completely silent with Google+ until they were ready to throw it open to everyone.
A small company like Facebook was can launch a product in niche areas, and grow it out, building hype slowly.
Google cannot do that, because everyone knows who they are, and cares about what they do next - I think, for them, launching a network effect product (e.g. not gmail, as while e-mail depends on network effects, g-mail doesnt) has to be all or nothing.
They need to open it ASAP, to capitalise on the coverage they are now getting, and get dense enough social graphs for the service to persist.