Asynchronous or one-way sharing is fundamentally incompatible with the current Facebook friends model. They'd have to rework a lot of the privacy code. Probably all of it, actually.
It would also be a major UX shift to bring Lists back to the foreground and make it a part of every interaction a la circles. And if they did, it would be hard to do it without offending existing users.
Facebook also may have big datacenters but perhaps not currently architected to handle videochatting at the volume that Hangouts require. Google has a one-up on them there since they've offered free video chatting for years, and they also have gotten many people to already install the Voice and Video plugin by offering free phone calls in GMail for the past two years. That plugin is key to a good videochatting UX in a browser (Flash by itself has some technical and UX problems with webcam access).