Probation officers, detectives and even potential employers regularly scan social networking sites such as MySpace and Facebook for information about their clients, suspects or applicants.
According to this article Lara Buys was about to get sentenced to probation for drunk driving that killed her passenger, but when the prosecutor saw a picture of her drinking at a party after the crash, he used it successfully to argue for 2 years in prison instead.
This article talks about teens who bragged about committing serial arsons. Of course law enforcement must verify who posted it, but this type of evidence seems to be successfully utilized in court, especially during the sentencing phase. And for writers, it just might add an interesting twist for your story.