Promoting content/features on social networks is only part of the job, good marketers manage to engage their audience and lean on it to push the message beyond an immediate circle of followers. The goal is to create a loyal community of fans that can tap into their networks for you to increase the reach of your brand.
2 major benefits of doing social media marketing:
1. Social Signals contribute to Search Engine Rankings - in this case shares and comments are valued more than simple likes, because engagement is harder than just voting.
2. Successful Viral Campaign can bring:
a) free valuable traffic to the site - traffic doesn't equal sales, but the goal is always sales or B
b) publicity for the brand - this is especially true with controversial or original content/message