The +1 has an indirect effect on your site’s search rank. This does not mean the more +1’s a link has, the higher rank it achieves in traditional search results. Take this scenario:
When a Google+ user +1’s a piece of content, he gives it his “stamp of approval.” Then, say one of his connections from Google+ searches for the same or related topic. Because of Search plus Your World, his friend is more likely to click on the same link the original user +1’d. This is because content recommended by friends and acquaintances is often more relevant than content from strangers, according to Google.
Since the +1′d link has a chance at a higher Click-Through-Rate (CTR), there is a greater potential the link will be shared, whether it be on Facebook, Twitter, Google+, or any social network. An experiment by Rand Fishkin, CEO and co-founder of SEOmoz, tested the relationship between Twitter and Facebook shares and search results in Google. He found a positive correlation between the number of retweets and shares a link received and its search ranking. This means, the more the link was passed around on Twitter and Facebook, the higher the search rank of the page. This in turn led to better SEO.
A Google +1 can indirectly lead to a better page rank. A greater number of +1’s increases a link’s potential for a high CTR, which could lead to increased social sharing, and in turn can increase its Google search rank. What’s important to note here is the correlation, not causation, between +1′s, other social shares, and search rank.
The bottom line is, the SEO effects of a +1 are very indirect, which means traditional SEO practices should not be ignored. SEO methods such as link building, relevant keywords, and URL structure have a more significant impact on page ranking.