IOS 4.3 compatible with 3gs?

So I have iPhone 3gs. I can't download iOS 4.3… Apparently 4.2.1 is the current one and up to date.
why is this? I have a friend who I'm sure as iOS 4.3 with iPhone 3gs?
Please help, I can't access things like facebook, instagram and whatsapp. I can't even access facebook via safari which is very strange.

Are you sure it is an iPhone 3GS? The 3GS can get up to the latest iOS 6.0.1 no problem. However the 3G can only get up to iOS 4.2.1. Sounds like you have a 3G not a 3GS.

You have a 3g not 3gs.
the 3gs can get ios 6.0.1

Probably a 3g. My mom has the 3gs and she is on IOS 6