2. They can mutate. Games update regularly, weeding out unused/disliked features and adding in better components that will help them "survive". The Challenge Missions in Mafia Wars are a prime example. The most recent was South Africa, meant to capitalize on the World Cup – the objective of this mission was to fix the matches by bribing referees and so on. Now that the World Cup is over, the new players drawn to the game will likely stay due to the fun they had with the "mutation".
3. They are acellular, that is, they contain no cytoplasm or cellular organelles. So far as I know. 'Cause if they did, that would be disturbing.
4. They carry out no metabolism on their own and must replicate using the host cell's metabolic machinery. Indeed, Facebook games are helpless to reproduce without the living "host cells", i.e. human beings. This is where all of that annoying "Please be my neighbor on FarmVille!" notification spam comes from.
Oh, and they're tricky. Tricksy little viruses they are. They even act benign, rewarding the host when they pass the contagion on, or requiring transmission of the virus to continue in the game. This usually comes right when they've got their hooks in the player. 'You need more mafia members to play this level. Recruit. Recruuuuuit!' And the player dutifully sends out request spam to everyone on their friends list, and some poor schmuck picks it up, and the cycle begins anew. These things snowflake out – and once you start with one game, others will follow."
-Alex Villanueva draws some eerie parallels between real viruses and the "viral" nature of social gaming