People decide what's appropriate for them to do in a situation by examining and following what others are doing.

Nobody wants to be the only idiot buying your product, so they look for proof that many other people use it and love it.

Social proof is one of the most effective ways for young startups to effectively reassure potential customers and build a brand. Lots of Facebook likes, tweets, +1's and other social media shares is a good indication of social proof.

Other ways include showing off the number of users or customers, showing case studies or logos of customers, linking to articles published in third-party sites and media outlets.