I understand you're looking for help with unblocking someone on Facebook. Since you mentioned "post," I'll assume you want to know how to unblock someone and perhaps share that process or announcement.
We live in an era of digital absolutes. When we delete a file, we empty the trash. When we unsubscribe from a newsletter, we vanish into the ether. But on Facebook, the "Block" button was never designed to be a delete key; it was designed as a fortress. unblock someone facebook