Yes, I think only looking at public posts skews the results. The G+ customer is more likely to be there for the purpose of fine tuning their audience, both incoming and outgoing. I know that only maybe one in 10 of my posts is done as public. People who want to spray their thoughts and opinions at the widest possible audience gravitate toward... the widest possible audience!
If you want a bullhorn you go to Twitter, because that's not really what G+ is about. G+ is about friends and interest-communities, news with decent media embedding, and just generally having a much better S/N ratio than Facebook or Twitter.