Currently Being ModeratedSep 8, 2012 8:47 AM (in response to nayan.thapa)
The official Facebook app requires iOS 4.3 or later so you cannot install that on your present iOS.
You have two options:
1. Update your iOS on your iPhone.
2. Go to the App Store and search on Facebook. There are numerous 3rd party apps that will allow posting to Facebook. Check the requirements of each to see if there is one that will work on the iOS that you have on your iPhone.