Facebook's userbase is still going strong though. Facebook also has a moat, since you cannot just move, because Facebook is where your friends are. With Slack, my company can decide to move and there are no other dependencies on anything external. We just move and that's it.

The moat is the integration points. The real differentiators for Slack are centralized history (better than IRC) and easy web endpoints for integration (better than pretty much anyone else). After a company invests significantly in a myriad of integration tools (both external and homegrown), they have little incentive to move. That's Slack's moat.

Still, they have no real presence in the consumer space and that makes them vulnerable to the first player that can really cater to both businesses and consumers. As it is, consumer vendors (MS/Skype, Facebook, Google) are fumbling in the biz space, but that will not last forever.

Edit: have a look at Slack 3rd-party apps for what i mean by integration. Because it's so easy to build them, there are tons.

When you say integration, I assume that goes beyond the chat bots that we have seen on other platforms since ages?

You forgot Yahoo Messenger and AIM. Before Snapchat it was iMessage, WhatsApp (still extremely popular).

ICQ -> MSN & Yahoo Messenger & AIM -> SMS (sidekick phone!) -> iMessage & Facebook (the age of iPhone) -> WhatsApp -> Snapchat

Hey, 56k was a real thing ;-)

x86 Assembly -> C -> C++ -> JavaScript

edit: Okay, I get why I'm being downvoted. :) But I just wanted to say, apparently in a vague way, that evolutions happen. People don't invest their precious time just because something is "hip". Apparently they solve a problem of their predecessor.

C++ to JS is a pretty huge jump. C++ -> Java -> JS might work better.

Hey you miss PHP, Ruby, Python. The new hotness is called Rust.

Don't forget the apparent JS -> Go

More like LISP -> ML's -> C++, Java, C#, Ocaml -> Rust, Julia, Clojure, and the other better ones. :P