I think the trees are cumbersome and messages are often missed unless I'm constantly checking the thread.  I like FB because they'll send you non-email notifications of replies to threads that even happened years ago.

Now I do agree that there should be a way to subscribe to a thread so you can tell if any resonded to it even if you didn't post on it. But I want nothing to do with a crappy face book like forum. It sucks and not in a good way. Long live the trees of TER!

I have no experience with Facebook so I can't speak to that but the "tree" is ok. I agree that it's cumbersome.
I like the flat format except that all the replies to a post not the OP's winds up at the bottom. There is no way to be sure what/who they are replying to. I wish there was a way for the flat form to be in the same sequence as the tree form. Actually I'm sure there is a way. It would just take someone that knows what they are doing to make it happen. The flat form works great with the Photo Theme threads on the boards where there is no commenting. Click flat and just scroll through the photos.