My impression has been the opposite, that facebook/myspace/etc are blocked more than webmail. The only recent survey I could find that touched on both social neworking and webmail is this one from august:

http://www.scansafe.com/content/employers-crack-down-social-...

"76% of companies are choosing to block social networking ... and Webmail (58%)"

Of course, if this thing caught on, presumably more companies would block gmail.