Android is architected to allow for "deeply intrusive" modifications to the standard Android homescreen. Manufacturers have already done this, as have independent developers like Go Launcher.
As an Android developer, I am happy that I am not seeing yet another Android fork like Kindle Fire, Nook, various Chinese forks etc. I prefer more HTC/Facebook people coming to Play as opposed to setting up yet another app store. If Facebook makes it easy to integrate Intents and such with this (they had some problems in the past), I am sure Android developers will be happy to integrate more with the Facebook API.
I think this is a smart move by Facebook. They started with problems on mobile - junky HTML5 apps which did not integrate well, but as time has gone on they have been making better decisions and doing smarter things. This is just one more good move on their part.