I was cynical about it to begin with, when it was just 'Facebook at Work' but I do believe it addresses a lot of the issues brought up in the blog post and the Workplace team are executing very well.
Accounts are completely separate from Facebook, no crossover at all.
I don't think it's a Slack killer, as they both solve different problems in different ways, and Workplace is very much a 'big co, top down' approach.
I do think other existing collaboration tools have a lot to be worried about between Slack & Workplace though.
Last stat I heard was there is over 14k companies using it.