Due to massive protests held in Egypt these days and the believe of the government that protesters use these social networks to organize themselves and to encourage people to protest.
Government however denied this banning and some media personnels said that this may be a load on these websites, yet these websites are perfectly working on every country on they world - except who are banning them as well !

It is also believed that BlackBerry services are also banned together with reports about cell phone and internet services being not well functioning.

really all of us was shocked because of what the Egyptian government did to ban these sites and to ban the Internet as awhole