Whatsapp has near universal market penetration globally, especially in countries where most people can’t afford iPhones and use iMessage.

Even in countries where iPhones are ubiquitous the cross-platform nature of Whatsapp and the fact that everyone from your friend to your grandmother is on it, makes it dominant.

Yes you could use Slack to message your friends like you would on Whatsapp. The experience won’t be the same, and setup will be a hurdle. But would you use Whatsapp to replace Slack at work?

I think the term “compete” is used very broadly here.