No need for free invision boards or anything like that. When this place crashed back in 2008, John set up
www.theflatearthsociety.net. That forum still exists, so if this place crashed tomorrow, everyone would just go over there instead. It's been largely defunct since the merge (board politics, long story), but it's always there, just in case.
By contrast, you only have to look at what happened to the original FES to see how a fragile a largely offline community would be. One fire essentially destroyed the Flat Earth Society - this forum really was a new beginning, starting from scratch. In contrast, it's hard to see this community being wrecked if something happened to the main site. Even if .net didn't exist, the dedicated members would find each other and set something up. We might lose some more casual members, but the society would survive.