The number may be crazy, but Slack does benefit from network effects. For example, my company has many bots/integrations with other systems, years of institutional knowledge, and connections with multiple external vendors.
Switching would be painful so there'd have to be some pretty compelling reasons. (And who's got time to recreate all our custom emojis? :-))
That's a moat, not network effects. A network effect would be if your friends at other companies get value from joining Slack while you're on it. But there's not (except maybe sharing integrations/bots).
I'm assuming that third parties are a lot more likely to join an external channel on a platform they're already using (and are more likely to be responsive as well).
I think all the existing integration are examples of an indirect network effect -- companies wouldn't invest in providing them if there weren't already users on the platform.
