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.

Indeed, most monopoly rulings have happened in markets with competing oligopolies.