The group I currently play with uses a group chat over SMS/MMS. But I'm going to be DMing a campaign with an open table, so I need something more persistent and flexible. I've used Discord in the past but IMHO it has way too many features.
What have you used to keep in touch with your players?
Discord. I got a channel for absentees, one for scheduling, one for talking, one for roleplaying, and of course vocal and webcam features. Also uses a bot for music on it for free. You can add how many channels you want for : lore, maps, links, memes, etc