Update as of May 20th, 2016 08:00 AM Pacific Time
We have identified the issue with Feed Posts and our team is currently working closely with Facebook to have it resolved at the earliest.
Meanwhile, as a work-around, we have made a small change to help you publish Feed Posts. All manual feed posts will appear as a browser pop-up instead of a Facebook overlay.
In other words, you will be required to allow all pop-ups from "https://apps.facebook.com" on your browser to be able to post feeds manually, until we fix this issue permanently. Please read below for detailed instructions.
Click here: http://zynga.my/FVCantPostToFB