"... as part of our ongoing efforts to improve Platform (sic) through policy and technology changes, applications are prohibited from dead-ending users at an invite-friends page, and must never again prompt for invites after the user has declined.
More generally, unnecessarily gating access to application features behind inviting friends is not a best practice...it is misleading to entice an investment of effort or promise a result and then -- without warning -- hold expected content hostage behind an invitation ransom; this is now expressly prohibited...."
This is a long overdue move on Facebook's part. Right now there's no way for Facebook to automatically enforce the new rules. They'll have to rely on users to report apps that are in violation. Most users probably won't do this, because they won't know that the apps are doing anything wrong; the precedent for forced invites has already been set.
It'll be interesting to see when (or if) things change.