(let me take my Privly hat off and put on my Machine Learning Researcher hat) Facebook needs users far more than access to users' private communications. Their "like" button and the demographic data they collect is more valuable than crawling messages between users. Privly could slightly impact the efficiency of advertising, but it won't shut down advertising as a business model. AdBlock poses a bigger risk for that.
(Privly hat back on) Our biggest challenge to legitimacy is interfacing with the web in a way that is private and allows sites like Facebook to "news feed" something only if the user reading the news feed can read the content. This is why we need to work towards a standard where both Facebook et al and Privacy can coexist. There are many options in this area, with differing amounts of privacy, but it is ultimately up to the posting user to post and the host site to decide what to do with it.