I recently bought an unlocked jail broke iphone 3g off of Ebay for my wife to use on Straight talk. Everything works fine, it's just the IOs is 4.2.1 which apparently is to out of date in order for her to use certain apps(mainly Facebook). Is it a to big of a head ache to fix this? Remember I'm a noob so treat me gently :)
Try to find(google) an older version of facebook, and installed it.
Unfortunately iOS 4.2.1 is the latest version of iOS supported on the iPhone 3G. It is somewhat difficult to install older versions of iOS applications once the vendor has updated them. Doing so involves hacking your device so that it can install cracked applications. Unfortunately, as the primary use for doing this is application piracy (even though that is NOT what you are attempting to do, and I want to be clear I know that's not what you're attempting to do), we cannot discuss the method you would need to use to install the app...
If you are able to find an older version of Facebook on the internet, with no modifications, it will fail to install on your device. Additionally, applications posted to the internet outside of the App Store can be modified versions that steal personal or sensitive information and passwords.
I apologize that I don't have a better answer for you than that there is a way to do it, but we unfortunately cannot detail how to do it.