Anybody who thinks they know for sure that slack can't possibly be worth $16B should go back and read some threads from around the Facebook IPO (like this one https://news.ycombinator.com/item?id=4320585 ). It's very easy to write reasonable sounding things - making accurate predictions about the future is much harder.
Well, I've been saying Snapchat can't possibly be worth $29bn when it did IPO. And surprise, it lost most of this value. Snapchat is a lot like Slack - it's a feature, not a product. Once Facebook copied the feature, Snapchat was doomed. Same will happen with Slack (in this case I think MSFT will be its undoer).
I really don't understand why companies like Snapchat and PayPal don't do the reverse - add Facebook-feature to their product! Surely Snapchat would be more useable if it had profiles, friends, etc. (i.e. if it became Facebook-lite, in other words, Instagram clone). Sure, (some) users will revolt, but you don't need to force it down their throat... make it opt-in, or privacy-preserving, or something, and make sure that it's genuinely useful!