You must already have become an "addict" or a "victim" of viral social games we find in Facebook. And you must already know that these games are a lot different from what you generally play on your computer or in other gaming websites; as Facebook games involve and engage your network of friends. Thanks to Facebook's own way of representing what you are doing on the platform, your entire network will know what you are playing, and on which level have you reached. This works as a stimulation to others and they feel an urge to indulge in the activities as well.
 
The infographic here talks about a number of reasons - why Facebook social games have become so popular?

These games can be "mindless" for certain sections, but is actually means of serious cash for game developers. Facebook has a track record of earning as much as $500 million only from games. So, games here are serious!