Our organization is mostly virtual - with most of our tech/dev team onsite at our headquarters in Austin.
As the VP of Marketing based in Laguna Beach, the majority of my internal comms are done on Slack with a Screen Hero integration. Also use Slack Calling with ease. We even have a #bs channel where we share silly memes, pics, quotes, and quirky news -- it has made a difference in our employee engagement.
We love it. It has been the best internal comms tool I have have ever used. I set up specific channels for activities and events and only invite those who are impacted by these activities to participate.
We are only a 30-person team and I love it.
The other companies I work with use Basecamp - which I also like -- but I love the Slack app on both Mobile, MacBook Pro and MacBook Air.
I am a multi-Slacker and will stay that way.