Your comment about looky-loos got me thinking about real-world socialization. People who hang around the edge of a group in the real world, looking like they're trying not to be noticed, get pegged as creepy - it looks like predator behavior in the wild! The group then has a reason to either invite them to participate or keep them out - otherwise they're making the space feel unsafe.
The edge-hangers have two good choices: They can be very visibly present and then join the conversation when an opportunity presents itself, or they can leave.
Social-media edge-hangers don't have to do anything, they can just hang there forever without clearly joining or leaving the group.
In in activist group there is always a concern, founded or not, that law enforcement, opposition groups, or opposed individuals will try to infilitrate the group. People who are in this situation [1] in regards to romantic attraction also are creepy as hell.
The edge-hangers have two good choices: They can be very visibly present and then join the conversation when an opportunity presents itself, or they can leave.
Social-media edge-hangers don't have to do anything, they can just hang there forever without clearly joining or leaving the group.