Both Google and Bing have admitted that rankings can be affected by social media links (aside from beneficial second-order effects on the link graph). This comes after months of suspicion from SEO experts that such was the case.
Basically, links coming in from Facebook, Twitter, and other social media sings can provide a substantial boost to any SEO effort. The development is definitely a step forward. But this doesn’t mean that you should go ahead and spam social sites starting today! There are a few things to keep in mind:

• Social Authority of a User is looked into – for links on Twitter to count, both Google and Bing looks at how many people follow you and how many people you follow. Numbers are important in this game. It adds heft and credibility to the links in question.
• Author Quality – this just means that search engines can associate an author with his offline persona especially if he’s a public figure. After all, name searches for them will usually be significant as well. It is important to take note that author authority is different from PageRank, however.
• Diversity of Sources – 50 tweets containing a link from a single account are not nearly as valuable as getting it from 50 unique accounts.
• Surrounding Content – the text that accompanies a link provides search engines with important information about the topic and relevance. It also acts as a substitute for the lack of anchor text (specifically on Twitter).
• Activity Level – basically the number of re-tweets, clicks, and likes (from Facebook) influences the weight of a link.
• Extent of the Network – how many friends and followers do you have? This can provide a significant boost to web authority. More is better with few exceptions (like inauthentic accounts and bots).
• Friends and Follower Ratio – on Twitter, reciprocity is important. For example, if you’re following 5,000 people and only 100 people are on your list, then your account is not as authoritative as an account with 5,000 followers.
• Association Bias – if you regularly tweet about your newest blog posts, it will be detected by the search engines.

There is certainly nothing wrong with that, of course, but it carries less weight than if an outside author or company promotes your link on social media websites.

These are just some of the things you should consider if you intend to use social media to benefit your search engine rankings.