Most Americans spend at least a few hours a week on social networks of one kind or another... for work, catching up on news, entertainment, artistic exploration, job searching or, well, just socializing.

Who are we spending time with on these platforms? To begin to answer this question, we decided to take a closer look at the demographics of the leading social media platforms. The statistics reveal some interesting findings about who's participating in these massive online communities.  Which sites boast the most diverse audience? The best educated? The most affluent?

Let's go to the numbers....

OVERALL SIZE

We don't need to remind you (but we will anyway) that the size of networks matters. It matters a lot. More people posting and sharing makes for a more lively, enticing experience. Sparsely populated networks die. The best measure of size is not the overall number of accounts, but the population that visits the site regularly. Depending on the set up of a particular network, people don't have to logged in with a membership account to participate actively in a site, or even have an account at all. We thus turned to the number of monthly unique visitors to gauge size.

According to data from Quantcast, Facebook and YouTube are neck-in-neck for the title of most visited social platform. The sites each have a different focus, of course --- YouTube being a prime spot to watch trending videos, Facebook being where you, eh, look up ex-girlfriends --- but both are seared into the fabric of most Americans' everyday lives.

How much so? The Land of Zuck averages about 143 million unique visitors in the United States every month, while YouTube logs 144 million uniques. By comparison, the U.S. population is currently estimated at 312 million. So nearly half of all Americans check in to Facebook and/or YouTube at least once each month, and often much more --- making the sites almost as popular as calling Mom.

The third most popular network is Twitter, with an average of 68.6 million visitors a month. This is less than half of Facebook's traffic, but still a massive community of people, no matter how you slice it. Considering the extra steps that someone needs to take to really embrace Twitter --- learning what @ symbols, RT, and DM signify, for instance --- the size of Twitter is all the more astounding. 

Business-oriented LinkedIn can clam 42.5 million unique monthly users in the U.S., while Flickr lately has averaged just under 18 million users.

SEX

If your Facebook newsfeed seems to feature more posts by women than men, you're probably not imagining it. Facebook's audience includes 15 million more women than men, which even at Facebook's massive size is a noteworthy female tilt (55% to 45%). Twitter also has a 55/45 female to male ratio, which means women are overrepresented on these networks by a few points. The national gender split in the U.S. is 50.7% female to 49.3% male, as of the 2010 Census.

Furthermore, according to a 2009 study by Facebook's in-house sociologist, Cameron Marlow, female Facebook users are uniformly more active on the site than their male counterparts. Marlow found that the average female Facebooker posts between 10 and 26 times a month on average, while males post between 7 and 17 times.

YouTube most closely mirrors the overall gender pattern of the country, while LinkedIn skews slightly male (51 to 49%) and Flickr slightly more so (53 to 47%) according to Quantcast's statistics. 

RACE & ETHNICITY

Compared to the U.S. population at large, all of the major social networks have the approximate racial and ethnic makeup that you'd expect. Where things get interesting is looking at which networks skew the most from the norm in the area of race: 

Twitter is a strikingly diverse platform, where a combined 31% of users are non-white, including 16% of the audience being identified as black. Compare this to YouTube, another hugely popular network where the minority audience makes up only 27% of the monthly visitors, the Quantcast data showed. 

Meanwhile, a full 75% of Facebook's overall monthly visitors, which adds up to well over 107 million Americans, are Caucasian. No wonder Facebook is the only one of the platforms to make the vaunted "Stuff White People Like" list. Hispanics are notably underrepresented on Facebook, the data suggests.

But ultimately it is Flickr that is perhaps the single most reflective social network in terms of matching up to the demographics of the United States at large. The percentage breakdowns for each ethnicity in the Flickr audience is generally within one or two percentage points of its general population counterpart.

LinkedIn, meanwhile, has the most overwhelmingly white audience of the five networks examined. Caucasians make up a staggering 83% of the professional network's audience, while African Americans comprise just 5% and Hispanics just 4%.

EDUCATION

Nearly half of the visitors to Facebook, Twitter, YouTube and Flickr lack a college degree. But before you rush off to condemn those kids who waste time on Facebook rather than homework, keep in mind that some of these social media users are still in high school or college, and thus are not degree holders yet. (Across the country, 40% of adults have at least some college education.)

LinkedIn is the standout here, with three-fourths of its users being college educated, which is not surprising given the site's clear job-hunting and networking focus. LinkedIn also has double the number of graduate-level educated users than any of the other networks --- 27% compared to between 13-14% for the others.

Our favorite stat about LinkedIn is that an estimated 5% of users are 17 years of age or younger. Rock on, all you young career-minded overachievers!

YouTube is the only social network where college grads don't outnumber non-grads --- they are split an even 50/50. 

INCOME

Somewhat predictably, LinkedIn's audience is both the oldest --- with 70% of users over the age of 35 --- and the most affluent. Over two-thirds of LinkedIn users make $60,000 a year or more, and nearly 40% of its users earn $100,000 a year or more. We could not find any data on how many are in the "millionaires and billionaires" category that have a tax hike coming their way.

YouTube had the lowest earning audience, no doubt skewed lower by its larger youth and student base. By contrast, Facebook has a relatively high-earning audience, with 62% of its monthly visitors last month earning $60,000 or more.

In all, the makeup of the largest social networks is by and large reflective of the U.S. as a whole.  We're eager to dive deeper into some of the questions raised in these demographic slices, chiefly why are so few blacks and Hispanics using LinkedIn? And will Twitter find a way to monetize its sizable female member base, which as everyone knows, is a marketer's dream.