Most people do not share anything. Nothing. If they have signed up for Twitter, it is to lurk or to read news. The idea that a social network could ease people into sharing more widely with the world their thoughts and ideas is what is so engaging about Google+.
You are right that those of us who have used Twitter for this aspect of Serendipity would be missing out on something if we only used Circles. But for the vast majority of Learners, Teachers, and Students have never been comfortable with sharing and are terrified of sharing things with the “public” (a la Twitter). We should encourage them to make circles and share within a team of dedicated and encouraging individuals. And we should encourage them to add us to their circles so that we can reshare. We should also show the benefits of sharing publicly. We should show how serendipitous it can be to look at those who have commented and add them to those whom we listen to and want to engage with.
One more thought:
I have found Google+ to be more serendipitous as of late because I cam able to “follow” people easier than I have ever been able to do on Twitter because I can do so just by hovering over their name. The conversations on a hashtag, generally don’t support learning so well as conversations that I am intentionally engaging in with people that I already know.
My learning is co-created, but I learn best with those that know they are challenging my thinking, not those that just want to push out a controversial opinion. Anyway, I could have posted a comment on the blog, but I thought Plus might be good way to continue the conversation too. I’ll link to the post in case people don’t know what I’m talking about.