As you may already know, one of the primary reasons why people use the internet is because of social media. It only becomes logical if search engines make use of these sources for their rankings.
Over time, it's becoming clear that the stronger presence you have in social media, the stronger your presence becomes in search engines as well, which makes it good for your Website SEO.
However, it should be noted that it is not clear how search engines take a look at things from Twitter and Facebook in order to help the organic searches.
Danny Sullivan, the Search Engine Land Chief posted a very important article about how social media affects search engine rankings and surprisingly enough, Google and Bing gave direct answers that they do, though they didn't exactly divulge their respective secrets as to how it does affect search engine rankings.
According to Google, when an article is retweeted in Twitter, it sort of flags up as a signal and they also consider how many people shared the articles. Despite of these links being nofollow links, it clearly shows that they in fact have bearing when it comes to Google searches.
The same can be said about how they treat Facebook Fan pages. The more authoritative the crowd sharing links to your content, the better you rank in the search pages.
Bing also explains that they also look at social authority of users. They also consider the number of people following those users and add a little more weight in the links they have. In other words, Bing decides when links carry more authority by determining the person who tweets it.
In terms of Facebook Fan pages, Bing explains that if the same links is shared in both Twitter and Facebook, it is more likely to be legitimate.
So, if you really want to rise in the SERPs, it may be good to looking into increasing your authority in both Twitter and Facebook.