I do a monthly draw on my Facebook page. Anyone who leaves a review gets put into a monthly draw for a $25 service credit. I had 52 at one point, then Facebook switched from reviews to recommendations and I lost them all. Back up to 30 now, all since October.
I find that even if people love you and refer you to their friends, they won't actually sit and write you a review unless they have some sort of incentive. Or maybe that's just my client base... I live in a university town, and all of my clients are students.