The research firm also revealed that Facebook and Twitter users are more likely to be based in the city.
"Nielsen's online data shows that about half of the US population visited a social networking website in the last year and that number grows every quarter," said Wils Corrigan, an associate vice president with Nielsen.
The survey, which studied the top seven social networking sites, showed that Facebook users generally have an "upscale profile".
Nielsen also said that people who are more affluent than the other two-thirds of the population are 25 percent more likely to use Facebook than those in the the lower third.
And conversely, those in the bottom third of the financial chart are 37 percent more likely to use MySpace than those in the top third.
Other recent studies have shown that as Facebook and Twitter grow in popularity , their user bases are growing older .
A July report released by iStrategyLabs shows that while the number of Facebook's US high school and college-age users declined over the past six months, its popularity among the 55-and-older crowd is booming.
In fact, the number of 55-and-older Facebook users showed staggering growth - 513.7 percent - in the last six months, the digital consulting firm said.
That means Facebook and Twitter, which both have shown phenomenal growth in the past year, apparently appeal to an older, wealthier demographic.
And with expectations mounting for Twitter executives to finally come out with a business plan by the end of the year , the demographics of the site's users will have to be factored into the equation.