For text sharing, apparently they recently made a modification preventing "app's side" to put in/force a text message.
You can share a picture, but not just post text to the user's wall.
I'm not completely clear on the exact nature of the modification, my colleagues did talk to me about it but we had to chose and modify the nature of sharing we were going with in our application.
This is not for Instant Games, but I would suspect the sharing issue is similar to the one we had because of this new change on FB's side.