> who wants to be on a marketplace or messaging app that doesn't have the most people?

But it doesn't have the most people. I have long lost that sense that FB is where everyone is. That feels so 2016

> Apple realized this with iMessage

In general iMessage is not a good example because it a product that leverages the unique position of Apple owning the hardware. No other company can play the same game. Apple don't want you to use iMessage on other systems, they deliberately make the experience worse for Android. But who else can afford that?

You definitely can seamlessly use Whatsapp, Viber, FB Messenger, Telegram and Signal all at the same time on all your devices.