I hear people mention Slack once in a while on various forums but I have no clue what it is. I do not know anyone who uses it so it's not something I, or anyone I know, feels a need for--otherwise we would be searching it out. And therein lies a problem. $16B for something few people need or want? I guess I have to look up Slack and see what it is.
EDIT: So I looked it up and it looks like something useful for a lot of remote workers that need constant, instant contact with people working on the same stuff at the same time. To me that's a pretty niche market. If I thought the phone or email wasn't good enough, I might pay $5/month for such a service. But irc works pretty good still today.
IRC isn't worth $16B