Interesting court case on tv used facebook as evidence

I'm sure I'm late to the game here but I was stuffing my face with pizza and diet coke with my BPA filled cup at a restaurant while they hypnotize me with TV in my face. Anyway, something that struck me as odd although I normally don't listen much to the star struck lawsuit tv shows was a lady suing an ex-roommate for property stolen during a move out debate. I'll leave the long story short but in the argument the plaintiff brought up a facebook conversation apparently between her and the defendant as evidence. The judge asked the defendant if she was aware of the tv or if she denies taking it and of course she denied it, then brought up the conversation which mentions the tv and the defendant apparently commented on it saying it was a loss or something. In the long run the plaintiff lost the case as she wasn't there during the incident, the defendant used the popular defense that it wasn't her on her facebook account arguing and was someone else and the judge said it was all circumstantial evidence so the case was lost. I'm just curious if someone here has legal experience but wouldn't a conversation on facebook have an expectation of privacy? Is that legally allowable in court if it's private email? I wasn't sure if since it's obviously related to the case if there's an exception or obviously if it was a public wall post then it was meant for the public to see and you wouldn't have any expectation of privacy. Since in the IT field we all are probably familiar with having to create disclaimers everywhere for expected use and no privacy, etc it's a curious line for email. Last I recall even though a person uses your computer (i.e. they don't own the equipment) they still have an expectation of privacy and their personal email shouldn't be monitored without consent. I believe universities end up with this conundrum often with personal computers attached to their network, etc. Does anyone here have legal experience for electronic monitoring or submission to court in this day and age? I know it's more likely just an opinion topic I just expected the judge to say something about the evidence not being admissible.