[rabidrat]

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).

[quicklyfrozen]

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.