Uh, nope. Most people use communication platforms due to network effects: because they were invited to use it, because their friends/coleagues use it. Slack is no different.
Considering Slack is a per-organization service with minimal cross talk between organizations, your premise seems flawed.
I mean, Slack appeared on scene after many other technologies existed. People recommend it for their friends because it works well. There is no global network effect at play.
I don't think his point needs Slack to have a global network effect built in. I'd assume that, usually, a small handful of people in an organization decide on using Slack. Once the choice is made, network effects lock members in. Assuming the average org has 20 people or so, probably 90% of Slack's users are probably thanks to network effects.
Uh, nope. Most people use communication platforms due to network effects: because they were invited to use it, because their friends/coleagues use it. Slack is no different.