Facebook and Twitter are now the largest social networks in the world. In fact, all the biggest brands and companies have a Facebook page and/or a Twitter account. Your product or service may not be as well-known as theirs but at least you have the privilege to make your product appear as it is.
When it comes to social media, the main thing that it comes down to is popularity. People judge your service based on how popular it is and the more likes and followers you have the more popular your product is.
Let me add on to say that likes and followers don't equal sales. But likes and followers can bring in sales. The popularity of your social networks gives the visitors a tendency to take your offer more seriously. Would you purchase a product on a Facebook page with only 100 likes or a Twitter page with only a couple of hundred followers? Of course not! But with a few thousand your prospect would check out what you have to offer. If everyone else likes it then it must be good.