This doesn't seem to work for Facebook. I just tried to download it for my old 3G and it said an upgrade to iOS 5 was required. It didn't give the option to download an older version.
Yep, it's been around for a good while. I definitely had it when I used to use the 3G regularly.
I'm sure devs can remove older versions that are incompatible with their backend if they wish to. This is probably why you can't download an older Facebook.
Okay, I've never used the Facebook app myself, but let's try to download it on my 3G…
Working! I get the same message about downloading an older version… Done! I wonder why it's not working for you mcneilis… I'm on the Australian App Store.