Discord nailed reliability, quality, and usability. Its tech is still not worth $10B, though, even if the tech needs to be as good as it is to keep attracting users.

Tangential: I really do fear for them if they get acquired, though. Their tech is built on the actor model, which almost nobody else uses or understands enough to build on. It feels like it's likely that if they merge with Microsoft they'll get rebuilt internally (probably to use Azure) or have their infrastructure replaced with Teams' infrastructure.