Facebook has become an outlet for customer complaints since the manufacturers or dealers are very quick to remedy something that could tarnish their online reputations. This is not a bad thing as long as checks and balances are in place. For instance, take the time and set up your privacy options on Facebook. This way no one will be able to see anything about you. One also needs to be mindful of the amount of information you post on social media when you complain. Rather than writing your whole story, write a short summary (without too many details) and send the rest using a message. This way the key information stays private.
As for the representatives from both LRJ and Mercedes, well...their attitude towards the situation was less than stellar.
Getting information from the dealerships themselves is also way too easy. I have many times phoned with a VIN number and shorty after received the whole history of the car, some times even showing owner information. While I had no intentions other than checking out the history, others could very well use this in a criminal way.
Last tip would be know your dealership and know the chain of command. Rather than complain on Facebook straight away first communicate with the sales or service managers. If that fails raise the issue to the DP. If you still feel that you are getting nowhere then you should consider social media.