@jrh3 Yes, it closed down about a year ago. They originally locked the forum for everyone could still read the posts, but no longer make any posts. They had a message that it was closed due to inactivity as "everyone is on Facebook" type of message. I guess they simply abandoned it completely and let the domain name lapse and go offline now.
I tried to pull it up with the Wayback Machine, but only the 2013 site here is working - so it is not going to show the closed message. It will however may be of some use to read older posts.
Update- I did get the front page to load somewhat, view here, and the message is "Sorry but the Forums are now read only. Another casualty of Face Book".
On Facebook they are called groups, for example our forum has a group here.
There are many on there you will have to do some searching. To access them sign in, look at your options under the Explore area called Groups (left-side of the page). They can also be found just by typing keywords like mantis or mantid into the search function at the top of any page.