Yeah. You just explained why I haven't talked to hardly anyone on Facebook chat anymore. I don't know that they're actively wanting to chat, just that they're on Facebook, possibly just passively browsing things. I still ping people on Facebook chat, but only if I need to ask them something or want to share something I discovered with them specifically. I used to chat with several people every night in my youth. I don't have quite as much time to do that anymore, but I do have some time. I'd love to know who actively wants to talk with people.

This explains why I rarely PM someone directly. With group chats you can just jump in and if someone wants to talk they will just join in but not feel pressured to join if they don't feel like it.

That's because you are assumed to be always online, always available, you being busy implies doing something else, which isn't in their model.