Do not run Promotions that offer potential rewards in exchange for +1ing content, following a user, adding a user to one’s circles, +mentioning a user (other than the contest creator or sponsor or an affiliate of the contest creator or sponsor), having other users enter the promotion, or voting in polls. Also, don’t run Promotions that encourage users to post unoriginal or repetitive content.
Since policy changes so often, I though I'd record it for future reference. Thanks to +Meirav M. for the heads up. I am including screenshots in case it gets edited again. Also, I will be that guy when I see this policy violated.
https://www.google.com/+/policy/contestspolicy.html