Always wondered what would happen if someone was to happen to have every valid US/CAN number in their contact list (all 3-4 billion), since WhatsApp doesn't validate you actually know the contact just that you have their phone number.
I suppose you could use that limit to set up enough WhatsApp accounts on proxies to effectively have access to all registered #s?
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!
Creating an app with the sole purpose of backdooring WhatsApp on a user's phone seems like it'd open you up to a lot of lawsuits. Ethically its a mite more questionable, but the original article is still unethical in that you're monitoring people without consent.
Like I said above, I'd do this just so that they'd crack down on it. It's still a "means justify the ends" argument, however, so you have to be quite comfortable with moral relativism.