Interesting article in the Tribune last week: http://www.chicagotribune.com/news/local/breaking/chibrknews-prosecutors-bragging-on-facebook-leads-to-charges-20110504,0,5797986.story
A woman bragging on Facebook about a battery she committed the night before at a nightclub leads to her being charged with Aggravated Battery. It should be rather obvious, but if you commit a crime and do not get caught, probably not a good idea to talk about it on Facebook!
In fact social networking sites are quickly becoming evidence in many criminal charges. I as a defense attorney have used information that I found on Facebook to impeach witnesses who are adverse to my clients. I have also seen people alleging harrassment or abuse on Facebook when seeking an order of protection. My advise is not to post anything on these sites. But if you must post, make sure that only your friends can view your posts and pictures.