Does Facebook even need Onavo for that? AFAIK any Android app can check which other apps are installed and the names of all other running processes, with little to no permissions. And since the main Facebook app is installed on most devices they can use that to gather the data. Not sure how accurate it is in comparison, but it definitely has a wider reach.
I guess the point is that owning Onavo gives them access to data about users who specifically are not actively using Facebook -- which is obviously the space where FB would be looking to do defensive acquisitions.
