There’s one denizen of the social networks that won’t show up among the avatars of your Twitter followers or among your friends on Facebook, though it’s there all the same — the U.S. Justice Department.

Seems Justice and some other federal agencies have been using social networks as a source of information on citizens, though how they do it, how often they do it, how much info they mine and what they do with it is a secret. That’s why the Electronic Frontier Foundation has sued the government to find out what agencies’ policies and practices are when it comes to data collection and surveillance using social networks such as Twitter and Facebook.

EFF sued only after the agencies refused to comply with its Freedom of Information Act requests within the mandatory 20-day period.

According to a article, EFF claims there have been incidents where law enforcement organizations have used socnets for investigations, though no one knows how often or how many:

The EFF said in its complaint that it is seeking the information to “help inform Congress and the public about the effect of such uses and purposes on citizens’ privacy rights and associated legal protections.”

It cited news articles that reported police searching Facebook photos for evidence of underage drinking and an FBI search of an individual’s home after the person sent messages on Twitter during the G-20 Summit notifying protesters of police movements.

The potential number of citizens who could find their social network invaded by cops and feds is huge — Facebook, the biggest socnet site in the world, has 300 million users, while Twitter boasts 58 million.