Onavo’s VPN allow Facebook to monitor user activity across apps, giving Facebook a big advantage in terms of spotting new trends across the larger mobile ecosystem. For example, Facebook gets an early heads up about apps that are becoming breakout hits; it can tell which are seeing slowing user growth; it sees which apps’ new features appear to be resonating with their users, and much more.
This is such a shame. I actually used to use Onavo back in the day when their VPN would actually crunch the size of images to save data on your plan. I heavily use Facebook Ads for clients (and sometimes myself) and this is not a good thing. No company should ever have access to everything a person does online.
Don’t believe me? Check out Gizmodo’s write up on the subject.