how can i post on facebook via ios?

If you're signed in to Facebook, go to Notification Center. You'll see a "Tap to Post" in the upper right.

Firsty, you should sign in your FaceBook account in "setting-Facebook", and then you can pick up a photo from photo and use sharing-Facebook, then you can post it via your iOS.

First you have to install the FB app! At least on my new i5 with iOS 6.0.2, all the FB notification, push configuration and privacy controls are there, but if I touch any one, it tells me to install the app. Which I have not done as I won't pollute the phone with that time consuming and battery devouring distraction, but I digress.

Same applies to Twitter. But the birdie is far less of a power hog.