Android: Facebook introduced a genuinely cool feature with Chat Heads, but there's one problem with it; you can open it anywhere, so per-app security PINs aren't very helpful. Chat Lock fixes this problem.
The app allows you to lock access to floating message apps like Chat Heads. With Chat Lock enabled, you'll need to enter a PIN or pattern in order to access or send messages. It doesn't deal with the few lines you can read from the notification, but if you want to keep out prying (or account hijacking) eyes, it's a nice barrier to have.
Chat Lock [Google Play Store]