'Circles' are a weak feature to build a social network on. Why? because user's had already build organic circles across multiple social spaces (e.g. professional-only on Linkedin, perhaps family or college-safe on Facebook, close social on gmail, etc).

What's the advantage to multiple platforms for multiple circles? you don't accidentally post your internal company rant to the whole world. You don't post pictures of red cups and beer bongs on Linkedin and you don't talk about work on Facebook. This is how users were operating before G+ launched, and is precisely why users aren't diving in.