I think this probably happens much more frequently than people may realize.
I once gave a doctor a bad Yelp review and was very careful to avoid stating anything that could be claimed as libel - I only stated very specific and accurate facts and my opinions of them. They still seemed to try to come after me for libel (but quickly gave up).
Interestingly, I had even used a throwaway email address and Yelp account. But, they still tracked me down. I'm guessing that maybe they had Yelp pull the server logs, but I don't really know.
My Yelp review was then hidden. And, the Doctor responded directly to my review, calling me out by name and essentially calling me a liar (I wasn't). I certainly didn't want to get messed up with a legal case, so I ignored it. However, it was a very startling experience.