Yesterday, we got a rare look at how information on your public social media profiles—including Twitter, Facebook and LinkedIn—is being harvested and resold by large consumer data companies.

Responding to a congressional query, nine data companies provided answers to a detailed set of questions about what kinds of information they collect about individual Americans, and where they get that data. 

Their responses, released Thursday, show that some companies record — and then resell — your screen names, web site addresses, interests, hometown and professional history, and how many friends or followers you have.

Some companies also collect and analyze information about users’ “tweets, posts, comments, likes, shares, and recommendations,” according to Epsilon, a consumer data company.  

While many of these details were already available on the data companies’ websites, the lawmakers used the letters as a chance to raise awareness about an industry that they said has largely “operated in the shadows.”

“Posting to Facebook should not also mean putting personal information into the hands of data reapers seeking to profit from details of consumers’ personal lives,” Massachusetts Rep. Edward J. Markey told ProPublica in an e-mailed statement.

“Users of social media want to share with friends, not enable the sale of their personal information to data miners.”

Companies that collect social network information said they only take what is publicly available, and that they follow the rules laid down by each social networking site.

Acxiom, one of the nation’s largest consumer data companies, said in its letter to lawmakers that it collects information about which social media sites individual people use, and “whether they are a heavy or a light user.”

The letter also says Acxiom tracks whether individuals “engage in social media activities such as signing onto fan pages or posting or viewing YouTube videos.”

The company said that it does not collect information about individual postings or lists of friends.

Data companies of course, do not stop with the information on Twitter, Facebook, and LinkedIn. Intelius, which offers everything from a reverse phone number look up to an employee screening service, said it also collects information from Blogspot, Wordpress, MySpace, and YouTube.

This information includes individual email addresses and screen names, web site addresses, interests, and professional history, Intelius said. It offers a “Social Network Search” on its website that allows you to enter someone’s name and see a record of social media URLs for that person.

Epsilon, a consumer data company that works with catalog and retail companies, said that it may use information about social media users’ “names, ages, genders, hometown locations, languages, and a numbers of social connections (e.g., friends or followers).”

It also works with information about “user interactions,” like what people tweet, post, share, recommend, or “like.”

But Epsilon said it does not connect this social media information with any other consumer information in its databases. Other companies, including Acxiom, include social media profile data as part of detailed profiles on individual consumers. 

Instead, Epsilon said, it uses information from social media sites to “provide companies with analytics insights” and “help them better understand and interact with their customers.”

Both Epsilon and Acxiom said they obtain information from third parties that specialize in collecting social network data.

Experian, the credit reporting company, said that its marketing services operation “does not collect data from social networks for the purpose of sharing such data with other entities.” It did not say whether or not it uses social network data collected by other companies. (Experian said the data used for its marketing services is “completely separate” from the data used for your individual credit report.)

Markey, the co-chair of the Bipartisan Congressional Privacy Caucus, and a group of other members of congress sent questions to nine companies this July, in response to a New York Times profile of Acxiom, one of America’s largest consumer data companies.

“The data brokers’ responses offer only a glimpse of the practices of an industry that has operated in the shadows for years,” the lawmakers said in a joint statement yesterday on Markey’s web site. “Many questions about how these data brokers operate have been left unanswered.”

“We continue to collaborate with the U.S. government and federal agencies to help broaden the understanding of our business practices and the enormous value that the industry creates for individuals and businesses alike. As such, we advocate for a conversation that balances privacy considerations as well as the benefits of the appropriate use of data,” Acxiom CEO Scott Howe said in an e-mailed statement.

“While we provided a thorough response to the Caucus’ request, we honored all client, partner and vendor confidentiality agreements. We remain focused on our mission to strengthen connections between people, businesses and their partners.”