Cannot mount /system rewritable

I have rooted my phone and changed /system/etc/hosts file to block Facebook(just for test). However, since I did it, I am not able to edit the hosts file again. As a result, I cannot visit Facebook from my phone! I've tried some hosts editors in Google Play but without a result. I've also tried to mount system rewriteable but it returns error ("Mount: Operation not permitted"). I've already rebooted the device, didn't help. I have no custom recovery installed. I've used Playstore apps that promised doing the remount, but mount -o remount,rw system has the same issue. The superuser app in place is the one from KingoRoot.