yelp pays "marketing assistants" to post. These marketing assistants do no disclose their identity on the site, so you don't know who is a "plant" and who is not.
check out this business week article
"http://www.businessweek.com/t echnology/content/dec2 006/tc20061207_915943. htm
"Here's how it works: To help get established in a new locale, Yelp recruits paid "marketing assistants," to promote the site not only through everyday interaction, but also by kicking off online discussions and adding comments to other people's reviews to encourage reviewers to keep up the good work. Essentially, they help make Yelp appear to be a vibrant and outgoing community in hopes that it will actually become one. In some cities, higher-level community managers handle some of those same tasks, but also coordinate social events."
Interesting to note in their very own FAQ
"Additionally, to combat review "spam" and ensure the trustworthiness of Yelp ratings we have a system that automatically suppresses a very small number of reviews written by folks that aren't a part of our vibrant local community"
Oh, I get it now. Spam from Yelp is OK, but other's are not. Nice!
Some good old fashioned community building for ya!