Facebook Feed dialog loses Youtube path and parameters? (iPhone SDK)

Trying to use the Feed dialog to post a Youtube link. I'm putting the video URL in as the "source" parameter, but it seems as though it's stripping off the watch?v=..开发者_运维百科. -- what's uploaded (in fact, what's visible already in the Feed dialog itself) is just "www.youtube.com". Is there a trick here I'm missing?

Okay, so: it looks like I was using the wrong URL for the source, but that wasn't the whole story; you need to set source and link and picture if you really want this to work. E.g.:

source  : http://www.youtube.com/v/r3zlp3Ok7F8
link    : http://www.youtube.com/watch?v=r3zlp3Ok7F8
picture : http://img.youtube.com/vi/r3zlp3Ok7F8/default.jpg

What I thought was the source is actually the link, and the link and the picture are both special URLs. Substitute the video ID in all three.