Facebook got its start at Harvard University and MySpace has cultivated its image as a tool for aspiring rock stars. No wonder the image of the social networker has tended to be younger than the reality. With a huge advertising target at stake, an in-depth demographic study of social network users shows they are older than you might think.

One of the latest studies was published by Royal Pingdom. The upshot is that Facebook and Twitter users, with an average age of 38 and 39 respectively, tend to fall into that most coveted of demographic slots: adults with a hefty disposable income. A whopping 72% of Facebookers have a household income of more than $50,000 and 17% top the $100,000 bracket, says a year-old study by Ignite Social Media.

With respect to other social networks, it should come as no surprise that Bebo and MySpace are the youngest, while LinkedIn and Classmates.com are the oldest.

The graying trend accelerated in the last six months of 2008, when Facebook saw a massive 276% increase in users between 35 and 54 years old, a demographic that now accounts for about 45% of all its users.

How did this happen? Charles Petersen in the New York Review of Books pinpoints and analyzes what he calls the “suburbanization” of Facebook:

The surprise came when Facebook… caught on with members of all classes and succeeded MySpace in early 2009 as the most popular social network in America… Facebook managed to beat MySpace… by appealing to what has become the largest new market for social networking: parents.

He continues:

Most parents avoided MySpace, ironically, for the same reasons that drew [Rupert] Murdoch to the site to begin with: its central place in the media category euphemistically known as “urban culture,” which dominates contemporary American broadcasting, despite the fact that more than half of Americans now live in the suburbs… Parents, unsurprisingly, were put off… While many teenagers may prefer to decorate their rooms with the paraphernalia of hip-hop and drugs – and still continue to use MySpace in huge numbers – their parents have chosen to live in the suburbs for safety, privacy, quiet, and architectural uniformity, qualities that Facebook alone was prepared to provide.

If this was a conscious decision on the part of Facebook, then it was a winning one. With more demographic studies arriving to confirm the “graying of Facebook,” marketing departments simply cannot afford to ignore Facebook’s most valuable resource – the people who use it.