http://stacktips.com/tutorials/android/how-to-get-list-of-in..., https://stackoverflow.com/questions/3304685/how-to-get-the-l...
For example the Facebook app is a curious one. IIRC it also asks the system to notify it when a package (any package) is installed or uninstalled: https://stackoverflow.com/questions/11246326/how-to-receivin...
I guess they can easily track the popularity of apps like Snapchat or WhatsApp. Geez, also, identify any apps that are "going viral" in popularity, and either buy the company, or squash them through imitation...