I'm on a couple of social Slacks. The most active is me + three others, all of whom live pretty close to each other, and we've been using it to stay in touch while not seeing each other in person and also while not blowing up phones with notifications (i.e., other IM mechanisms will ping us in real time).

I think we mostly ended up on Slack because we already use it in other contexts - if we didn't, we might have gone for some other IM system.