Symantec security researchers say the main objective of the Flashback Trojan is revenue generation through an ad-clicking component. The botnet is shrinking at a rate of close to a hundred thousand machines a week as Mac users get around to download Apple’s tool for disinfecting their machines or installing antivirus. Google’s advertising clients are paying for Flashback’s attackers to host ads on Google. Ultimately, Google doesn’t know someone has clicked into its client’s ad, and the client never knows its ad wasn’t delivered.
Source: https://thehackernews.com/2012/05/flashback-malware-creater-earning-10000.html