Uptime aside, you also have to consider the effect of a single point of failure when you self-host. If your in-house communication hub goes down when your site does, it's going to make firefighting that much worse and you'll pay for it in a longer outage.

Here at FB, a lot of day to day coordination takes place via FB products. But production and release engineering communication happens over IRC, especially during major outages. The fallback factor in critical to keeping the plane in the air.