Using links is the best way for search engine to determine relevancy but social signals are starting to be included as search engine ranking factors.
With link spam, no follow links, and paid links becoming more prevalent, links have become less trustworthy.
Enter social networks. Sharing a web page on a social network can be seen as another way of voting for the content on a given web page. Social signals are continuing to rise as an important ranking factor that search engines are using.
If all links are not equal nor are all social networking accounts. The search engines are getting better at ranking the reputation of an individual social networking account.
This is mainly an attempt to stop spamming of the search engine. Otherwise what is to stop someone from creating fake accounts and sharing votes for a given web site?
The search engines rank social network accounts with a higher number of connections to other reputable accounts and who share varied content regularly as having a better social reputation.
Just like links, quality is better then quantity but being widely shared on social networks can be helpful.
It is recommend that if you don’t have a Twitter, Facebook, Google+, or LinkedIn account you should consider creating them and posting the relevant sharing widgets on your web pages.
Google has another method of voting for pages called Google +1. Google has stated this is used as a search engine optimization factor in ranking your page so you should consider adding this service to your pages as well.
On the other side Microsoft, which owns the Bing search engine, also has a stake in Facebook. Better include a Facebook “like button” while your are at it.