The Privacy Act of 1974 does regulate how the government uses information about citizens, and it allows people to find out what the government knows about them and has done with that information. But the act does not apply to data collected by outside companies, such as social networks. This issue has come up in recent years because government agencies increasingly use data brokers like ChoicePoint for various sorts of investigations that are not subject to Privacy Act protection. There have been proposals to broaden the act to account for new technologies, but none has gotten much interest from Congress so far.

This article mostly just repeats paranoia about government using social networking sites, but its still worth reading for the legal context.