Employees don't make decisions of choosing "what tool I would like to use", otherwise a ton of non-tech employees may even go as far as using things like Facebo

Employees don't make decisions of choosing "what tool I would like to use", otherwise a ton of non-tech employees may even go as far as using things like Facebook chat. The original OP actually provides the real answer (under the greying of the words). CIOs/CTOs at bigger companies make the decisions that cost money (evaluate risk when depending on external communications tools), so with the right network and the right investors, Slacks high-level sales team managed to get the guys at the top to buy into their product and thus, even less than 1000 organizations gives them 500,000 paying customers per month.