One thing that Facebook does rather poorly, and forums such as A2K excel at, is deeper conversations. On FB, they are often lost in a sea of post/reply/etc. I can't hit reply to a reply on my phone; it goes to the initial post and so the thread of conversation gets tangled.
Also, the algorithm does not often pull up your most recent conversation. I have particularly found this with large groups or pages. The discussion is buried, and the notification doesn't even get me back to the correct discussion URL within the group or page.
Facebook basically needs to improve their search function (which will be a large Manhattan Project-style issue for them, I predict), as it misses a lot. This will be tough for them, as so much content is created so quickly, and by so many different outlets. If they could make some sort of a "you were last here" link (there is your history; you can find it within your profile, but that is often imperfect), that would help a lot in terms of sustained conversation.
Although for FB, sustained conversations are likely not within their plans or their overall strategy. After all, they don't really want people talking, except in the context of promoting products.