The banner advertisement for greeting cards is intermittently displayed with an application called Farm Town, which has more than 9 million monthly users according to information published on Facebook.
If the bad Shockwave Flash advertisement is displayed, the user is redirected from Facebook through several domains and ends up on a Web site selling fake antivirus software, said Sandi Hardmeier, who studies malicious advertisements and blogged about the issue.(See also “How to Remove Fake AV Software.“)
Source: Yahoo! News: Software News