Google launched Social Search in 2009 and since then we have seen results from Twitter, Flickr, Quora and other social media sites appear along with Google’s standard search results. In the latest improvement to Social Search, Google is now including Google+ posts.
Social Search features links shared by your friends on the web based on the relevancy to your search. An annotation will show up under the link stating where the link appears and which of your friends shared it.
In order to see Google+ posts in your search results you must be signed into a Google account and the posts must be public.
Via: SEO.com