I have often wondered that and to be honest it wouldn't surpise me, I know of quite a few large companies who pay at least one person to trawl the net and just look for references to the company and how the company is portrayed on the net so Nissan doing it wouldn't be hard to imagine. If nothing else they'd probably be aware of the site to ensure there is no copyright breaching etc. even a tv station like SBS with limited money can put out cease and desist letters to people using their name wrong so for someone like Nissan worldwide I'd reckon it's nearly certain.
Getting them to listen is another thing, however it does make you wonder about people who get warranties knocked back, especially if they have been voicing opinions on the net.