We are using Slack for this course. Our fabulous librarian used Slack with the Newbery group and invited me to join in. So I had a slight leg up when I was asked to use it for this. I was thinking about using it for author study so it has been a great opportunity to learn more about how it works. BTW, the kids tweeted a bunch of authors last week. Hope we hear back from them.
Here is an article about how one school is using Slack.