If I'm not mistaken, the Share buttons says:
Know someone who can answer? Share a link to this question via email, Google+, Twitter, or Facebook.
So, we are encouraging that you share the link around to get people to answer the question and if they like create an account (they will get a temporary account cookie based).
We already have a Google+ Page, Chat, Meta, and IRC Channel, and several others. I don't see how adding a Google+ Community will really. In addition due to trademark policies we'll never be able to have an "Official Ask Ubuntu Community" on G+ since it's a Stack Exchange property (hence the Ask Ubuntu Google+ Fan page). IMO it's best to just join the Ubuntu ...
I want to add to what Braiam has said. Stack Exchange has the following badges as reward for sharing a link with your networks:
Shared a link to a question that was visited by 25 unique IP addresses.
Shared a link to a question that was visited by 300 unique IP addresses.
Shared a link to a question ...