Five men believed to be responsible for spreading the notorious Koobface computer worm on Facebook and other social networks — and pocketing several million dollars from online schemes — are hiding in St. Petersburg, Russia, according to investigators at Facebook and several independent computer security researchers. Beginning in July 2008, the gang enticed Web users with invitations to watch a funny or sexy video. Those curious enough to click the link got a message to update their computer’s Flash software, which began the download of the Koobface malware. Victims’ computers were then drafted into a “botnet,” or network of infected PCs, exploited by the criminals for their improper purposes. The security software firm Kaspersky Labs has estimated the network includes 400,000 to 800,000 PCs worldwide at its height in 2010. Victims are often unaware their machines have been compromised.