Have you been a member of a Xenforo forum that closed? Why did it close and have you ever tried to save a forum from closing or took in their members to your own forum? Most of the Xenforo forums I was on that closed were admin-type forums such as XenFans, XFShowcase, XenAdmins and XFDriveby. I have closed two of my own forums, BlackGamer and Xen-SEO. I have members on my site 8th.us from two xenforo forums that closed, LurkingGlass and Desitamashah. In all these cases, the webmaster closed it themselves. It seems to me that forums only lasts as long as the webmaster gives a ship. I guess thats the main difference between forums and sites like Facebook and Twitter because if the owners give up, they won't close it, someone else takes over.
I used to own a general Canine Chat site started on vB and moved it to XF before i sold it.. The project never got off the ground really based primarily on the simple fact that it was to general and tried to cover to much ground to soon.. The chosen Admin didnt really help as all she could be bothered to do was copy articles from other sources.. I only recently went back to check the place out and discovered they had closed the doors after a 4 year effort.. The place somehow became a spam magnet from day one and got more spam registrations on any given day then it seen legit registrations in any given month..