http://en.wikipedia.org/wiki/Section_230_of_the_Communications_Decency_Act
Communications Decency Act States that:
No provider or user of an interactive computer service shall be treated as the publisher or speaker of any information provided by another information content provider.
So unless the website is manipulating the content, it is generally not liable for third party content.
However, there still lawsuits of this kind against websites. While large companies like yelp and myspace have in house consulors to deal with this type of issues, small businesses often realize it's hard to find resouces to deal with it.
