How far can the iphone 3g be updated?

I would like to be able to use the facebook app but it requires 4.3 i'm currently on 4.2.1 will not update any further through normal methods

4.2.1.is the lettering on the back grey or silver if grey.4.2.1 if silver 6.1

That's the latest version buddy I'm afraid, the support for the iPhone 3g stopped a while back now.

4.2.1 is the last. No further.