Why not block them directly at router level?
Most larger companies lease IP ranges.. Facebook and Myspace are no exception.
Code:
NetRange: 204.15.20.0 - 204.15.23.255
CIDR: 204.15.20.0/22
NetName: TFBNET1
NetRange: 69.63.176.0 - 69.63.191.255
CIDR: 69.63.176.0/20
NetName: TFBNET2
NetRange: 204.16.32.0 - 204.16.35.255
CIDR: 204.16.32.0/22
NetName: MYSPA-1
NetRange: 216.178.32.0 - 216.178.47.255
CIDR: 216.178.32.0/20
NetName: MYSPA-2
NetRange: 63.135.80.0 - 63.135.95.255
CIDR: 63.135.80.0/20
NetName: MYSPA-3
That's all I can find, there might be more.. but who knows.. unfettered access can always be achieved via tunnelling or by proxy.
Kinda makes filtering rather pointless.