Slack's been incredibly successful of late. You might not think of it as a 'social network', but it does most of what Facebook at Work is advertising itself as.
Slack is not positioned as a social network but as a productivity and collaboration tool, a role it fulfils really well.
If I had to choose I would pick Slack over Facebook at Work any day of the week.
Slack is great for quick-reaction and coordinated team org, but not suggest features, walk people through non-trivial decision, structure posts and comments.