Unless whatsapp did something to wreck their competition in Brazil, any effective monopoly they possess is by virtue of people choosing to use their system over other systems.
This sort of effective monopoly cannot and ought not be stopped unless the monopoly power is abusing their position. And how would you when it's a choice people have made? Mandate that every 5th person mustn't use WhatsApp?
Monopoly isn't even about market size it's about companies exploiting their market position using anti-competitive practices. There is no indication WhatsApp has done any of this except being the most popular product in a highly competitive and open market.
Principally, a monopoly is precisely about market share. Anti-trust and monopoly laws exist for situations where such monopolies are created by unethical or illegal means or when they abuse their monopoly position (directly with ability to charge customers for products and services, or indirectly by somehow shutting down competition).
And that's where, as you point out, WhatsApp hasn't done anything wrong. Even if they have a de facto monopoly in Brazil, unless they've done something other than what I've seen in the US, it's almost certainly the result of network effects and nothing to do with illicit practices.
With at least Claro whatsapp is free there[1], not sure about the other telcos. And yes, people use it. I just spent a few weeks there and for every call there were like 50 whatsapp-messages.
Unless whatsapp did something to wreck their competition in Brazil, any effective monopoly they possess is by virtue of people choosing to use their system over other systems.
This sort of effective monopoly cannot and ought not be stopped unless the monopoly power is abusing their position. And how would you when it's a choice people have made? Mandate that every 5th person mustn't use WhatsApp?