There are many different reasons to choose a service provider. Being FOSS should factor in, but it should hardly be the major factor. Currently, Slack and Github are better choices (Slack is far easier to get started using than IRC, and Github currently has more of a network effect than anything out there). While yes, it would be nice for open source projects to use open source solutions, they should be choosing the best tools for the job. If those aren't open source solutions, so be it.