Using the Graph API, how do I share a friend's post on my own wall ? I don't want to copy the original post's text and create a new post, becuase I need my friend to see that I shared his post.
Sharing a post from the API isn't possible - you can only re-post it using the same information, but it would effectively be a new post and wouldn't say who it was shared from (unless you add it manually in the caption, e.g. Someone shared ... via Someone).