This alleged virus has only been described on a couple of blogs, such as this one, but we've found no reports about it on sites of security firms such as McAfee or Symantec.
The virus supposedly acts in the following way: if you try to access a Facebook application located at apps.facebook.com/fancheck/ (previously called StalkerCheck), you'll get this message:
"FanCheck is adding new features and new capacity. This could take a few days.
Please become a fan, and we'll send you an update when we're back online"
According to sources, merely becoming a fan will "infect" your Facebook account, which is highly unlikely. All other reports about the virus seem to be created by spammers themselves. Google lists the phrase "facebook fan check virus" as a popular trend, but there lies the problem; if you actually try to search for this phrase, you'll be bombarded by sites containing malware.
We believe that this is merely a two part hoax: on one hand, you have a defunct application that allegedly lets you see who's been visiting your Facebook profile - which cannot work due to Facebook's policies, and all applications claiming to do so are scams. On the other, spammers and malicious hackers are feeding the rumors around this application to lead people to search for a solution, and getting their computers infected by malware in return.