I had an experience with Facebook locking my account once, and I came to suspect that they were doing it to punish me for logging in on a mobile browser to promote the app. The UX really seemed to be designed to be painful - interfaces where the buttons were just out of reach unless you scrolled a particular way, scary language implying it was unsafe to use the site that way. I felt like someone was getting a bonus based on how many people they could push off the browser into using the app.