Why does it make less sense to let it run independently (at least for now)? A team of 50 people are running a service that serves 400M MAU. That's impressive

Why does it make less sense to let it run independently (at least for now)? A team of 50 people are running a service that serves 400M MAU. That's impressive by any stretch and when you factor in that all of those are mobile users that's a compelling case for a company that purportedly had trouble figuring out it's mobile strategy. I guess FB is in the market for any large scale mobile apps (first photos, now messaging) to ensure they have a dominant foothold on mobile.