[smcin]

Marketing executives often have it as one of their KPI.

Clearly the less fuzzy way to define it is "How many people have you recommended this product/service to in the last n months?" not "Would you, if asked?"

Also, whether customers talk to each other (directly, privately) is not the only thing, there are online reviews, resources like HN, etc. And it's always hard to tell what's compensated or not, e.g. Snowflake's "grassroots" testimonial campaign on LinkedIn, testimonials etc., with the huge budget.