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Whatapp is a good platform, but unless you do lots of international texting (and your carrier bills you for it) its not worth paying for, since all the services it provide's you already have.
Instead of charging its user base, it could have easily sold user generated data about frequency of use, originating location and destination, peak times by area, broken down by age group.
There are many other valuable metrics that they could use against their information to make more than $1/year/sub without pulling a bait and switch on their user base.