What every custom launcher app needs (or maybe just Facebook Home)
A persistent "Nooooo... what happened? I want my phone the way it was before!!" uninstall notification in the notification shade that goes away after a trial period of say 7 days.
Based on my own personal experience, almost no one customises their Android phone beyond wallpapers and ringtones. They also never go to, or never want to go into, settings for their phone because they don't want to break their phone. It is really only the 1% who visit tech websites and forums who know about and have tried or currently use custom launcher / home replacement apps like Nova, Apex, Go, Action, etc. That of course is all set to change when the millions and millions of people on Facebook with Android phones start seeing the prompts to download the new Facebook Home app.
Most people who install a custom launcher, but especially Facebook Home, are going to be presented with a very different user experience - one that they'll either love or be totally freaked out by when it "takes over" their phone. These people need an easy and easily discoverable way to undo the installation. I think a notification is the best way to go about it.