When you are in the YouTube app, or however you're accessing YouTube, choose the share option and email. When the email comes up, select and copy the link from the body of the email. Cancel and delete the draft.
Go to the FaceBook app or site.
On your friend's wall, choose Write Post, and paste the link into the text section. It will automatically detect the link and provide a preview. Best to paste the link after you've said whatever you want to say, at the end.