Brian Fung writes:
When Google hands over e-mail records to the government, it includes basic envelope information, or metadata, that reveals the names and e-mail addresses of senders and recipients in your account. The feds can then mine that information for patterns that might be useful in a law-enforcement investigation.
What kind of relationships do they see in an average account? Thanks to the researchers at theMassachusetts Institute of Technology Media Lab, now you can find out. They’ve developed a tool called Immersion that taps into your Gmail and displays the results as an interactive graphic.
Read more on National Journal, and if you use Gmail, try plotting your own data. I stopped using Gmail a few years ago over privacy concerns, so I can’t offer you mine. 
Thanks to Joe Cadillic for this link.