There are plenty of reasons to watch what you say on social networking sites, but litigation is probably not the one you considered before flaming your boss on your Wall.
The first thing I do after I receive a copy of an employee-filed complaint -- before I read the complaint -- is check the plaintiff-employee out on Facebook and other social networking sites.
I print any information that employee has made publicly available. I save any pictures the employee has published online and I send a list of the employee's friends to my employer-client to cross-check against a list of current and former employees. I do this because, generally, a Facebook user will allow friends greater access to online content.
Why do I want this information? Because many social media users do not filter what they publish online -- they find social media cathartic. So, for every couple of banal "I'm going to the movies with John tonight" online posts, you'll find an "I just had the worst day in the office because …" post.
It may be considered unfair to have your words used against you in the court of law, but remember that one of the lessons from the JournoList flap is that what you say on the internet is never private and cxan always come back to bite you. That's why you should think about it before hitting the send button (I've even had people say intemperate things in a comment and delete it, not knowing I still saw what they'd sent).