With the steady growth of social networking sites – particularly Facebook, LinkedIn and Twitter – the characteristics and behaviors of their users continue to evolve.

The Pew Research Center’s Internet & American Life Project has been studying these changes in recent years through conducting a variety of consumer research surveys, and its lateest findings have just been released. And some of these key findings are quite revealing.

For starters, Pew finds that nearly eight in ten Americans are now using the Internet. Of these, nearly 60% are also using at least one social media site. And social media users now skew more heavily female (~56%), which represents something of a shift in recent years.

The Pew research also finds that among those people who engage with social media sites, Facebook is the 500 pound gorilla; more than nine in ten respondents reported that they are on Facebook, compared to only ~18% who are on LinkedIn and an even smaller ~13% who are on Twitter.

Moreover, engagement with Facebook is at a higher level. About half of the Facebook users report that they are on Facebook every day. By contrast, only one-third of Twitter users engage with that social media platform on a daily basis.

The Pew study also found that the average number of Facebook friends a user has is nearly 230 – a figure that frankly surprised me a bit. What constitutes “friends” break down as follows:

 Friends from high school: ~22%
 Extended family members: ~12%
 Coworkers: ~10%
 Friends from college: ~9%
 Immediate family members: ~8%
 People from affinity groups: ~7%
 Neighbors: ~2%

Interesting, on average about 10% of Facebook users’ friends are people that they’ve never actually met, or met only once.

Another interesting finding from the Pew survey is that Facebook users tend to be more trusting of others and more active in the extent of their social interaction on a personal level. This would seem to refute the notion that Facebookers may be more susceptible to pursue “cyber” relationships in lieu of old-fashioned personal relationships. To the contrary, the Pew report observes:

“The likelihood of an American experiencing a deficit in social support, having less exposure to diverse others, not being able to consider opposing points of view, being untrusting, or otherwise being disengaged from their community and American society generally is unlikely to be a result of how they use technology.”

And what about LinkedIn? Clearly, it operates on a completely different plane than Facebook and even Twitter. It has become the de facto Human Resources clearinghouse on the Web … an employment fair on steroids.

LinkedIn’s unique position in the social media sphere is reflected in characteristics like the educational level of its users. Whereas only ~20% of Facebook users have a four-year college degree – and just ~15% have post-graduate education – those percentages on LinkedIn are ~37% and ~38% respectively. (Twitter’s educational demographics are nearly identical to Facebook’s.)

LinkedIn’s age demographics also tend to skew older. This means is that even though LinkedIn users may not be engaging with the platform on a daily basis — in fact, only ~6% do so according to the Pew survey — they do represent a highly attractive professional audience that offers good potential for many companies in marketing their products and services.

Additional information on the Pew Research survey findings is available here. Check it out and see if your own social media behaviors mirror the Pew market findings.