http://google.com/safebrowsing/diagnostic?site=/
You will get a page back with how many pages where scanned and how much malware it found.
For facebook.com I got these results:
Of the 131,557 pages we have in the past 90 days on the site have been tested, have 31 page (s) without user consent malicious software downloaded and installed. The last time Google visited this site was on 08/08/2010. The last time suspicious content was found on this site was on 08/08/2010. Malicious software includes 132 scripting exploit (s), 3 trojan (s), 2 exploit (s)
It also mentions a bunch of domains like abeermahmoud.jeeran.com, albetaqa.jeeran.com, imageshack.us, rmooosh.net, textstream.co.za, freedesignlogo.com, and a bunch of URLs like facebook.com/dogswxeunck, facebook.com/pages/samra-iraq/imam-medhi-/85996831974/, and pdashmedia.com
I personally think it might be a good idea to have a look at which domains your users are going to, look it up and use this information to filter out the bad stuff.
