The brave new world is sorting out what companies, services, and communication mediums are subject to Common Carrier regulations. If Facebook is a common carrier, then there should be some expectation of privacy. If not, then not.
Facebook (and many service providers) are currently and deliberately in a gray zone. If they are not common carriers then they can do whatever they please with the goods (electrons, bits) that they transport because it is their own private property once you hand it to them; per the terms of service. That is good for business because people are handing over "free" stuff that the companies can turn into profits.
However, if companies are not common carriers and they own whatever is handed to them then they are subject to intellectual property violations, libel suits, fourth amendment oddities, and other violation of the law. A telephone company is not criminally prosecuted when land lines are used to break laws; a common carrier is immune to prosecution for what is transmitted. The lawsuits resulting from not being a common carrier could be bad for business.
In the long run, the market could sort this out. If some companies clearly are common carriers and some are not then consumers can decide. Or, it can stay muddled long enough for the gray area to become its own class according to judicial precedent, law, and the public.