Communities and circles are two different things
When you post something in a Public community, those posts can be seen by all the members in that community and also by anyone who checks the community, or searches for something on Google+ or Google and you post comes up in the search. Only community members can comment or share community posts, the non-members can only give Plusones to the posts.
If you have chosen to show the community posts on your profile, anyone looking at your profile will see those Public community posts.
How ever if people in your circles are not members of the community, they don't see those posts in their home stream. Only if they look at your profile or the community itself.
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I enjoyed the take this article had on social media interaction. Many of the "likes" and "shares" are fleeting thoughts and there is, probably, a decent percent of people that forget moments after the interaction is done. Marketers need to find better ways to draw customers in and create a meaningful interaction that customers will retain.