A virus is a piece of information that exploits a security vulnerability in an existing complex system, tricking the system into propagating the virus to its own detriment. Remember the word originally only applied where the target was a living cell. The usage where the target is a digital computer might have been considered a metaphor, but the deep structural similarity outweighs the difference in substrate, sufficiently to merit considering the new usage a valid extension.
Zynga 'games' are viruses that exploit security vulnerabilities in the human mind. The extension of usage is just as defensible as the original extension to digital computers.
terrorism is a "systematic use of violence and intimidation to achieve some goal" (dictionary.com). Zynga 'games' are acts of terrorism that attack the human mind to achieve their goals.
Zynga 'games' are viruses that exploit security vulnerabilities in the human mind. The extension of usage is just as defensible as the original extension to digital computers.