  Communication is essential in software engineering. Especially in distributed
open-source teams, communication needs to be supported by channels including
mailing lists, forums, issue trackers, and chat systems. Yet, we do not have a
clear understanding of which communication channels stakeholders in open-source
projects use. In this study, we fill the knowledge gap by investigating a
statistically representative sample of 400 GitHub projects. We discover the
used communication channels by regular expressions on project data. We show
that (1) half of the GitHub projects use observable communication channels; (2)
GitHub Issues, e-mail addresses, and the modern chat system Gitter are the most
common channels; (3) mailing lists are only in place five and have a lower
market share than all modern chat systems combined.
