This is a cross-post of my answer to a question posted on Quora:
Zynga’s approach is to leverage its scale, organic retention, virality, metrics, and infrastructure.
Zynga spends more than most gaming companies to acquire users (its acquisition costs are currently estimated at an average $120/user). Once a user is acquired, Zynga makes a significant effort to retain the user in the product (through in-product retention tactics and game mechanics) and in its portfolio of games (through cross-game promotions).
It also embraces virality as an acquisition tool to acquire “free” users through through its current users in-game (for example, when Facebook created its real-time streaming feature, Zynga was one of the first to design a communications platform around it to ensure that game play status updates would appear).
An added advantage is its custom infrastructure to leverage data. Zynga built many of its own systems including its own ad bidding system for Facebook to maximize ROI on paid social ads. Zynga also built its own analytics and data systems to capture and leverage over 1 pentabyte of data daily on its games.