Brand advocate
From DigitalJargonBuster
A customer who actively promotes a brand.
Brand advocates are more than just loyal customers because they actively promote the brand to other customers or prospects. Typically this happens with no incentives, and purely because they are fans of the brand and have formed a strong emotional connection with it. In the digital marketing environment where the barriers to the flow of information and suggestions are much weaker, brand advocates can be an incredibly powerful part of a marketing strategy.