Most people might not give telephone metadata – the numbers you dial, the length of your calls – a second thought. Some government officials probably view it as similarly trivial, which is why this information can be obtained without a warrant.
But a new analysis by Stanford computer scientists shows that it is possible to identify a person's private information – such as health details – from metadata alone. Additionally, following metadata "hops" from one person's communications can involve thousands of other people.
http://phys.org/news/2016-05-scientists-metadata-reveal-surprisingly-sensitive.html