We have a morning stand-up and a quick (10-15 minute) touch base session in the afternoon. Most questions that require multiple participants get answered then. We use Skype during the day for quick questions and an occasional Skype call for something a little more in depth. Seems to work fairly well for our small team.
I've only played with Slack a couple of times but I think I'm a little too easily distracted and it would drive me crazy. I guess it depends on the people and the teams involved.