You are talking about Facebook’s learning curve. Yes. Facebook could have a high learning curve but they have 1.94 Billion MAU who know how to use the platform. So when these users get on-boarded on Workplace, they know how the platform works making it easy for them to start using it rather than trying to learn it.
This is where Slack might have trouble. The new users they on-board will have to learn about mentions, channels, commands and more. It does take a while to play around and learn, which is difficult for non-techies.