Probably close to 80% of my Facebook activities these days take place in group messages for exactly this reason. It's easy to add and remove people to/from a g

Probably close to 80% of my Facebook activities these days take place in group messages for exactly this reason. It's easy to add and remove people to/from a group message while still retaining context, so it works well for planning ad-hoc outings where the group of people involved is quite fluid in its membership.