Based on feedbacks, Google has rolled out +1 recommendation to all users. This new feature of the +1 button in platform preview when user hover over a +1 button, they will see recommendations for other content which are available on your site. Talking about relevancy and on-topic, it will always refer to pages on the similar domain or subdomain as the +1 button. Recommendations will work across all +1 buttons for all users, whether or not they’re signed into Google+. The update will go live for all users in the next few weeks. If you want to see recommendations today, sign up for the developer preview.
(Image Source: Google Blogspot)