There's no better way than artificial scarcity to ensure accelerated adoption.
Plus, replacing the Home screen and interacting with the system at a lower level than probably e.g. Apple on iOS would allow, given the wide variety of Android versions (and hacks) out there that might be incompatible in unforeseen ways, Zuck probably does not want his company to go down in history as the one that (at least temporarily for Joe Avg. Users) inadvertently bricked a hundred million phones or so.
when he manages to remove every trace of phone-home crap in there, then it's maybe news worth mentioning
Then how would it work?
As a "leech-only client to Facebook" for the few who do actually care about their own privacy, but are nosy enough to want to read up on everybody else's every move (from the phones of all those to whom it couldn't matter less as they use an unpatched very verbose version).