A launcher is just another app; it doesn't have any extra powers. It just happens be the app that is launched/restored when the home button is pressed -- that's it.
It also replaces the app drawer, which means when you click an app icon to launch an app -- from either the home screen or the app drawer -- FB Home can make a note of it and send that information to FB.
There's obvious privacy issues, you don't want say, your employer finding out you just used certain apps like a job seeker or something else that posts to Facebook that you use it and ergo Facebook automatically likes it for you like they do with keywords.
