The idea being you incentive WhatsApp users to install your app that then harvests all their contacts and collates the "last seen" info on all of them. If they delete your app, you setup a proxy to imitate their device and continue the monitoring. Have a privacy policy that is super strong but has a couple "loopholes" that one can drive a truck through.

Is that the idea? Seems doable if you're not too risk averse, have no family and live in a country with weak extradition laws. Kidding, there's nothing illegal about any of this stuff or FB, Google and lots of other companies would not be in business.

FB would have a civil claim against you -- they paid several billion dollars for the legal right to all that user data!