I really didn't expect my post to be so controversial, is the HN crowd really so terrified about running there own hardware?
Luckily, there are a ton of great free and paid options out there these days!
99.99% uptime means it can be down for a few minutes a month, so all it needs to do is fail over properly. In practice, it will probably have many more than 4 9’s.
I think the real reason slack does well is ease of client + service setup, the brain-dead UI, lots of feature creep that a few people care about, mobile clients, etc, etc.
I’m not a huge fan, but it could be worse. At least they didn’t leak everyone’s password like hipchat did.