I think that the people leaving also knew this all along, but now they are finally coming to terms with the impact that FB has really had on people, the world, privacy, and the way it has desensitized everyone to this type of behavior.
Facebook hasn't changed, that's not why people are leaving. They're leaving because they've finally been able to look in the mirror and realize that this isn't worth the paycheck anymore. That's gonna be a tough pill to swallow for most.