This is not a monopoly, and monopolies themselves are not illegal. You mention antitrust laws but I am not sure what anticompetitive behavior is taking place here? The C&D itself is quite laughable, but speaking more generally, why should the network effect be regulated? Plenty of other communications networks are open, so any network effect of "walled gardens" ought to be attributed to the superiority of the product.
There are so many methods of communication available today, calling any one of them a monopoly is a bit absurd. You may not like what WhatsApp are doing, but that's not reason for more regulation...that's reason to switch products. But just because you don't like the alternatives isn't a valid argument for increased regulation.