I have an iOS 4.2.1 device. You can use facebook with the built-in Safari web browser.
Earlier iOS software is becoming increasingly difficult to find. You may want to look at VintApps, new in the Apple app store, that makes this much easier.
Although you will have thousands of available apps to choose from, they will definitively not be the latest greatest.
I wrote my 99 cent app specifically to help solve this problem. I need to add that "I may receive some form of compensation, financial or otherwise, from my recommendation or link."