probably many forums went to the more limited Facebook thread model which is mostly flat and only allows one quote level.
Facebook allows multiple levels, you can look at urls that have at least three (maybe four) --- nnnnnnn.
Oh, you are talking about quote levels, understood.
Facebook is really a totally different design. It is good for collaborative studies with friends, not so good for readable, enjoyable forums. I often end up mirroring what is written on Facebook to a 'real' forum

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Thanks so much, Matt. Good job.
My own research forum purebibleforum (full url click on my pic, then to About) similarly left vBulletin a year ago. Xenforo was recommended to me by a German Christian friend who also helped me set up on the super-economy Linode platform (which does need a little geek help.) I found Georg through the Xenforo forums.
The move to Xenforo is fantabulous.
Note that I miss the ability to have a subject text for a post. Xenforo decided not to bother with that feature, and I doubt it will be added. There is pretty robust discussion in Xenforo forum land about add-ons, features, etc. I noticed that the editor toodbar is a bit different here than on mine, curious why.
Overall, very pleased!
Granted you had tons of stuff in vBulletin that was customized, but they really did lose the thread. Xenforo was actually begun by some of their lead developers, who I understand got frustrated and wanted to start fresh.
Steven Avery