Social proof is the knock on effect of when humans see other humans doing something and thus become more interested. What Rand explains in this Whiteboard Friday is the more intimate and personal the nature of the social proof, the greater the effect.
Give specific data to the target demographic, as to how many people within their demographic use your product or service.
Social media is a powerful tool for social proof. It is more powerful social proof if someone within your social network gives something credit. It raises its esteem in the users mind.
In person social proof, face-to-face is the pinnacle of effective social proof.