OpenDNS has released a report [warning: PDF file] on the white- and blacklisting of domains in 2010. Interestingly, Facebook was the single most commonly blocked site – but it was also the second most commonly whitelisted site. This means that networks that disallowed social networking sites in general were still likely to make an exception for Facebook, likely owing to its popularity as a legitimate marketing tool.
Apparently the bad guys were also aware of this. The report indicates that Facebook is the second most commonly spoofed site for phishing attacks, behind only Paypal.