Are you tracking campaigns in your CRM? Not sure with others, but in SFDC, you can adjust campaigns to attach advocates to them, when needed (if they were speakers at an event/webinar, or gave a quote, etc). By doing this, you can track the pipeline influence of that campaign and attribute a % of it to your advocates, thus showing their contribution. You can also add a checkbox to opportunities to understand if an advocate/a quote/a story was used in the decision-making process and build attribution from there too.