Remember Facebook's reason for existence? It was finding people you know or lost touch with. But that's not enough for Facebook any longer. Just saying you know someone isn't enough for Facebook to wow its advertisers now. After all, Facebook has no idea why you know this person, how often you talk, message or meet up, and above all it has no way of evaluating a particular link on its social graph. Messaging not only gives 'weight' to links in the social graph that Facebook maintains but also offers the opportunity to scan messages and tailor advertising.
The genius of messaging isn't simply in the real-time advertising possibilities but in its ability to separate the wheat from the chaff in Facebook's network. The headline grabbing claim of 500 million users is not all it's cracked up to be for Facebook. When Zuckerberg tries to woo advertisers, potential investors and in general promote the value of his enterprise, it isn't the 500 million that gets him the advertising cash but the number of active accounts and how much Facebook knows about each individual and how they are all connected. An account without frequent updates is useless to Facebook and worse still, although negligible on an individual basis, it represents a cost to the firm for storing data.
Messaging, so long as the system is not beset by spam, is a profitable way for Facebook to pinpoint high value accounts and more importantly analyse high value links in the social graph. Suddenly Facebook is able to realise that out of the hundreds of so-called Facebook friends, the ones you communicate with over its messaging system are the ones that matter to you. After all, performing the 20th century task of actually writing a message to someone must mean that person has some important relationship with you. Don't be surprised if Facebook offers some sort of voice over IP communication system to further allow it to track phone call interactions in the future - it's the logical progression.
Scanning email messages to glean information for advertisers sounds like a very questionable practice, but the truth is that Facebook is half a decade behind the firm that pioneered the technology. Google freely admitted to the practice when it launched Gmail back in 2004 and that did little to dampen popular enthusiasm for the service, which at the time was the first to offer significant storage space. The carrot of having a whole gigabyte for email storage coupled with the slogan, "users should never have to file or delete a message" was an instant winner. Facebook simply goes one step further, playing on the public's total disregard for privacy in the pursuit of convenience.