As of now it is simply Facebook with a different colour and focused on a group. Nothing special.
But I do think that Facebook will end up releasing a better collaboration feature to help integrate teams into Facebook At Work.
The reason I love Slack currently is the various apps and addons you can add, like the Github bot that posts when a pull request is created, or the Trello one that posts when someone completes a task.
If FB adds something like this (or better), I think they will grab a large portion of the market that isn't already dedicated to Slack.
Then it will slowly be improving features to steal Slack users after that.