The cited reasons vary, but all revolve around lost productivity. Of those that don't prohibit use of social networking sites, a substantial portion allow limited use, such as for business reasons only or "limited personal use." Altogether, only 10% of a relatively large pool of CIO's said they allow unrestricted access to social networking sites. How long will that 10% last?
There are issues that go beyond the impact on productivity though, as social networking sites are being identified as security risks. Network administrators worry that a user could infect a machine through careless browsing. That is certainly not limited to social networking sites, but the concern arises given their sheer size, thus making them a target. What's worse is that users typically trust sites like Facebook and that means they are more likely to become victims of malicious acts while browsing them.