It seems to be a problem having installed the latest Facebook app update (v29). Removing it “fixes” the problem.
https://developers.facebook.com/bugs/1632385646995079/
https://developers.facebook.com/bugs/962985360399542/
Update (Jun. 3, 2015)
Well. It seems that the new Facebook policy says that prefilling a message through setInitialText: is a prefill violation.
https://developers.facebook.com/docs/apps/review/prefill
So I guess the only way to share content from now on is the FBSDKShareDialog
https://developers.facebook.com/docs/sharing/ios