Advocacy, trust, referrals and testimonials
Advocacy is somewhat like loyalty: if you have to ask for it, then you probably won’t get it – and you might even create the opposite effect.
The next-best-things to advocacy are probably referrals and testimonials, because you can certainly ask for those. If the answer is positive, you may incidentally create advocates. If the answer is negative, you have an opportunity to devote some TLC to those accounts.