They have multiple metrics for most prolific, from complements , to reviews, to friends to followers. There is yelp elite status which allows users to filter for just elite reviews and elites get invited to special elite events.
Related are what I call achievement systems or interesting thing is ways to keep your most prolific users entertained. Usually cosmetic rewards (a different symbol, a larger number) for jumping through hoops.
Some examples:
-World of Warcraft (larger number for doing random tasks like killing 10 bunnies within 1 second)
-Call of Duty (you level up from 1-55 in the game, but at 55 you can choose to reset your level to 1 for a different symbol that allows other users to know you are hardcore and have reset your level up to x times)
-FullTilt Poker (rewards for completing a number of random tasks like folding two pair on the flop)
-Kongregate.com is a great example (you earn achievements for playing games)
Creating ways for your most prolific users to show to others that this is "their house" is a great way to keep them coming back.