Google offers users and companies who set up a Google+ page, build circles, and post content a boost in SEO results. For instance a search for ‘Mexican Food’ in my neighborhood returns a special inset (in black) with local businesses which sell Mexican food. These special results are before anything else.
A Google+ page allows you to gather your followers, broadcast messages and such. While content was once the most important factor in SEO, Google wants more and more access to your social graph. That is to custom taylor your search results to your interests.
The leaders in Social networking, Facebook and Twitter, only share data with Microsoft Bing which also individualizes search results with input from social data. Although Google is by far the number one search engine in the US and the world, Bing has been gaining market share in the US (19%) recently faster than Google.
A more elaborate discussion by Ricardo Carreira is hereby