I think in most corporates (outside the software industry) it's the other way around. I can't access gmail/hotmail but can access social networks at work (in finance).
Not a great data point, but interesting none the less.
Next thing we could expect is a Farmville in Gmail.
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.