Google preparing content recommendation feature on +1 button. When you hover the mouse cursor on +1 button you can see the recommendation for other related content from that site. This feature is very useful for site owners it may slightly increase their site page views.If you’ve already added the +1 button to your site, there’s nothing more you need to do recommendation feature will work automatically on your site, when it is available for all. Recommendations will work across all +1 buttons for all users, whether or not they’re signed into Google+.
This update will go live to all of your users in the next few weeks. Now it is available only those who joined in Developer Preview Group. If you want +1 recommendations feature right now sign up for the Developer Preview Group.