I'm curious about the legality of offering these under an open source license, since I'd assume that the original content isn't in the public domain yet? Unless Nintendo's TOS had placed these under that license itself?
If facebook shuts down it doesn't mean all multimedia content that was ever hosted there suddenly gets dumped into the public domain. At most you gave the company that run the site an irrevocable, international license to that content (depending on their TOS), it doesn't apply to everyone else and it didn't get put into the public domain or under some other open source license.
Nintendo is sending all users their post history so they can personally archive it if they wish. If anyone else archives it, it's just copyright infringement...
Personally, I respect the work that the Internet Archive and other archivers do, so this is definitely not a hill I'm willing to die on.
It's an interesting conflict -- there is a benefit to saving things in case of site failure, but simultaneously it is a problem on a copyright basis, particularly for written works where the author themselves feels small next to the Internet Archive. They've walked a fine, fine line with various items like DOS abandonware games where the owners are potentially in more of a position to "lawyer up."