A short and typically informative article from eMarketer: Brand Campaigns Drive Most Social Media Following.
This study makes the point that if you want more people to click the Like button, you better get out there and invite them to.
And from following a great link from that same article, more about what people want from Facebook and Twitter: What Social Followers Want
A lot are looking for special offers and deals. Less – but still a significant number – want interesting content. And some people are happy to support companies they buy from.