The Two Types of Viralities

When hears about content going "viral" this actually could mean two different things. The true type of virality, when people perpetuate a meme without any kind of reward, is known as "evangelism". The second type of virality, when people perpetuate a meme for some sort of reward, is known as "obligation". For example a obligation model for a gaming site may tie ability to play the game in exchange for placing an add in the players social feed. From a business perspective both types of virality are valid models, but typically the "evangelism" model is the most effective and successful. The problem is that with evagelism, you actually need to have a remarkable meme. Obligation viral models can work when others are not used to seeing the type of message you are sharing. But obligation virality doesn't create the same type of respect and perpetuating power as evangelism virality.

http://whatgamesare.com/2011/11/the-two-viralities-marketing.html