How recent is "very recently"? I've been hearing ads in the podcasts I listen to for something close to a year. Prior to that, slack was only something I had seen in HN titles, and even then I had never once clicked on one.
I like slack, I think it's a great product. On finally watching their video and installing it, I immediately saw the draw, it's basically IRC with lots of usability enhancements and a lot of easy-to-configure bots available with a click or two. That said, it took me months or ads to finally take a look, but I did because the advertisements let me know that it was a possible solution for the problem I was dealing with.
It may have spread because they built a good product, but I think it's equally important that they actually exposed a lot of people to information about it. Your great product will die if nobody ever sees how great it is.
