Social Media Platform is not a marketing platform; it is a place where businesses form a bond with their target audience. Users of social media websites generally visit your website to see what is hot and trending, they do not have the intention to buy your products or services. Think about it like this, if a user had the intention to buy a product, he would visit the marketplace directly, instead of coming to a social media website.
Secondly, social media websites offer too many options and content. Thus, users in such websites do not have a very long attention span. As soon as they find another thing of interest, they move on.