Well, it's not like it's a technological marvel or anything (I mean, the amount of ram it uses is kinda insane), and chat apps are like the "hello world" of network programming, BUT! From what I've seen the social effects of it are huge. There's just a lot of nice subtleties that they nailed, and it's so invisible that there just doesn't feel like there's a need to find an alternative. It just kinda works and you don't have to think about it.