I was confused by the "+1" button in Google Plus. I just spent an hour trying to figure it out. For anyone else similarly confused here is what I discovered.
+1 is NOT used for a recommender system to indicate "show me more postings like this".
+1 is primarily used as a simple means of providing feedback to the author that you appreciated their posting. It is primarily just a counter.
However, it also has some aspects of the share button: "If the post was shared with people in your extended circles who also have you in their circles, they may also see your +1 in their streams". Extended circles are your "circle's circles", that is your friend's friends. I think this means if You and YourEnemy have a CommonFriend, and YourEnemy includes You in their circles, but You don't include them, YourEnemy will be able to see your +1 activity (at least on public posts). Google doesn't make it clear if YourEnemy will simply see the +1'd post, or if they will be able to see that You +1'd it.
Lastly. if you +1 a product or brand (what exactly defines these isn't clear) then Google may use your name and image in ads. This can be opted out. For me the default was opted out, but that may be because I use Google Apps for Business.
I find it all very confusing (and I find Facebook even more confusing). Why can't they separate out "thank", "share", and "endorse"?