Each app developer decides whether to add share options, or not. When they are there, they usually show up under the action/share icon, It looks like a box with and arrow curving out of it.
FaceBook is now more closely integrated with iOS 6, so expect to see it show up in more apps.
Other than that, you can often tap and hold on a link to get a copy option. You can then go to another app and paste that link into whatever you are posting.