Slack is definitely a better option than Facebook at Work in my opinion. Social networks are a time suck at best and marketing one to an enterprise is simply inviting cynicism and negativity from those people that actually have work to do (obviously almost everyone except HR).

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.