IRC, the traditional method used in the shop lost.
After switching, for the first few months, the membership was largely the same. Now the community has grown. I think this is because HipChat and Slack offer:
1. Easier for people to use. The best IRC interfaces still aren't as nice as Slack and HipChat
2. With the interface, are the integration out of the box. For example, pasting a funny GIF from imgur is embedded right in the chat. This builds the community, as people feel more relaxed and willing to share. For developers merge requests, etc appear in chat.