Pretty scary article highlighting the results of a Stanford study looking at metadata.
Specifically, the study encouraged volunteers who also used Facebook to install an app called MetaPhone on their Android phones. The app was designed to act as a sort of slimmed-down version of the National Security Agency by attempting to gather the same metadata collected by telecom firms, and in turn, intelligence agencies. Volunteers who chose to participate allowed the researchers access to their calling and texting data, the date and time, and the duration of the call.
The results they found are disturbing. Just with phone metadata they were able to
“We did not anticipate finding much evidence one way or the other, however, since the MetaPhone participant population is small, and participants only provide a few months of phone activity on average. We were wrong. We found that phone metadata is unambiguously sensitive, even in a small population and over a short time window. We were able to infer medical conditions, firearm ownership, and more, using solely phone metadata.”