The actual state of affairs is a bit more complicated than most replies here seem to indicate.
Viber is blocked by some providers and by some wifi installations. Most of the time it works.
Facebook is not blocked per se, but the powers that be sometimes like to play with the DNS, so you cannot get at it without knowing either the IP address of the Facebook host or you need to change to a DNS not controlled by the authorities. Google or OpenDNS are two such possibilities.
Skype and FaceTime usually work fine.
So yes, things may work, but then all of a sudden they don't.