As I've commented before: G+ is to be judged differently.

Public post counts are not reliable measures, besides it's way too early to tell, ask again when it's visible to all users and integrated into all of Google's products and have the basic features of search and even "vanity" urls, not to mention removing the invite bottleneck.

It's Google's social play, something it must have, a list of people you know and interact with over Google products. It's a work in progress and will still unfold for some time.

And for the record if any other startup got the numbers they have it would be dubbed a huge success, it's just that people like to speculate about and judge everything Google as it gets them clicks.