I doubt you could sue a review site, otherwise all the big companies would be sueing every big review site which will inevitably have bad reviews of their products.
I suspect that it would be up to the company to prove the reviews are false and then request the review site to take them down, and only if the review site refuses, they would be liable for a suit. Or if the reviews were used to defame a person specifically (e.g. by claiming a manager was a rapist or whatever).
Im totally all for some property review site, I have been renting for the past few years and dealing with dodgy properties/agents/landlords have been extremely frustrating.