High powered PR firm Weber Shandwick sort of coined the term in some recent research they did on consumers.
This from Marketing Magazine:
High-Intensity Advocates are critical to reach.
The most vital Advocates are ‘High-Intensity Advocates’, who represent a small nine percent of all global consumers. They more actively influence others’ decisions, make decisions faster and demonstrate their support or lack of support more zealously than ‘Low-Intensity Advocates’ (36 percent).
Weber Shandwick also found that Advocates contain an important segment of ‘Badvocates’. Badvocates actively make their dissatisfaction known in a variety of ways and do so more quickly today than two or three years ago. They waste little time acting – 76 percent report expressing displeasure within one week.
An outline of Weber Shandwick's services include the following: "In addition to helping our clients identify and cultivate advocates, we also assist them in working with and protecting themselves from Badvocates."
However, several marketing research sites have pointed out that the study from which these conclusions were drawn was a survey of only 538 people worldwide.