I have used both options quite intensively for more than six months each (both Facebook internal tools at Facebook, and Facebook at Work at Facebook; I am probably one of the very few who has and has no direct financial interest in either company) and Facebook at Work is significantly richer in features than Slack — notably longer posts that allow to explain, argue, compare. It is useful up to the point where meeting (outside of creative explorations) become non-essential.

Slack is great for quick-reaction and coordinated team org, but not suggest features, walk people through non-trivial decision, structure posts and comments.