There are several service I can think of that have a large user base and support for free (github comes to mind). However, I have to play devil's advocate and say that even some paid services have horrible (or non-existent) support and service.
> How can a company like Slack survive if it gives away resources like this?
Perhaps what should have happened was that slack should have worked out a deal with this group to display advertising. If the quotes of their user base was correct - I'm sure any advertising group would drool over that kind of user presence. However, this could spin out of control with the wrong manager with dollar signs as their eyes.