I don't think a launcher app like Facebook home can be associated with the Android home button automatically when it is installed, so the browser toolbar comparison isn't perfect.
Automatically, no. But it will give you a prompt that will allow you to set the home button to go to Facebook Home.
With different home apps, the user is always in control. (Although resetting a default once you've chosen it could be made a lot easier)
So comparing it to browser toolbars doesn't really work.
I imagine most customers reset the default simply by uninstalling the new launcher. The way through the app settings is really well hidden.