Discord is often called “Slack for Gamers”. It has channels similar to Slack. It lacks the threading feature. It has additional community oriented features like custom emojis. It's generally more casual and playful in it's design. It has a focus on voice and video chat and allows for “always on” voice chat rooms that people can casually drop in and out of on a whim.
This might be useful for a more diffused activist group but generally would not be recommended for serious work.