facebook has removed the ability to post on friends wall using graph api from 6th feb,2013. checkout the official doc here http://developers.facebook.com/roadmap/completed-changes/ under subsection Removing ability to post to friends walls via Graph API from February 6, 2013 changes
update
Alternatives you can try are
- you may use feed dialog to post on friends wall, that will prompt your user and if that user confirms, it will get posted on friends wall.
- if you don't want to prompt your user, then you can post on your user wall with their friends tagged in the post then those post will also appear in friends wall assuming friends approve the tag.