I get what you're saying but I'm not so sure -given it's team-centric (read: closed to outsiders) model - qualifies as network effect. That is I don't really benefit from anyone else on the "network" other than my team. So if your team doesn't want to Slack the friction here is so low (a need to setup your account) that the benefits (e.g., control, privacy, etc.) can be justified.

In addition the communication "niche" is MASSIVE. Communicate, as humans it's what we do. There's a reason why FB paid soooo much for WhatsApp, yes?

If add-on can make this product something between Slack and (the shitty implement of) FB groups, there's plenty of room for such a thing.