I think there are two distinct questions here, viz. is Social useful at Work and will Facebook at Work "fail" like Yammer or crush/be-crushed-by Slack et al. The reason these are distinct questions is that Facebook at Work is likely to be much more than just a Social Network and intrude heavily into Slack-like territory with Group chats etc. We wrote an article on this recently that explores this issue: https://tmail21.com/blog/is-social-the-future-of-work/
