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.

My first thought was Slack must have turned down Facebook's offer, thank the gods. It's not too weird to ask coworkers to sign up for Slack, but it would be really weird and probably unacceptable to ask someone who works for you to sign up for Facebook for all kinds of reasons. I'd feel more comfortable requiring employees to use Reddit at work, seriously.

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.

Or IRC.

Atlassian Confluence is also great. It's built around planning and idea/document management but has a lot of social elements like a company-wide questions board, a blog, notifications (@user), etc.