If I were leading Slack, I'd be ecstatic. This move by Facebook is validating.

Also, I'm sure they'll produce lots of insights and make lots of mistakes I can learn from...on their dime.

I disagree. This is great for Slack. They already have a product that people love, and of course this will raise their profile immeasurably, as people hear about FB's play and look into the available options.

Additionally, Slack now has just about the best tagline in the world: 'We're not Facebook'.

No, this is going to destroy Facebook.

Picture it this way:

  1. You show up to work, and Facebook is now mandatory. 
     It's attached to your real name, and Facebook 
     helpfully connects your real profile to all your 
     co-workers.

  2. You now have to delete ALL of your non-work contacts, 
     for fear of gossip contamination, and NEVER use 
     Facebook outside of work again.

  3. Everyone who has Facebook at work, now needs something 
     to fill the gaping void of recreational social media.

  4. A new upstart fills the void, and Facebook crumbles and
     implodes.

Maybe not, but it might eat into a lot of document/collaboration platforms.

Disagree 100% - have you used Slack?