It goes a lot deeper than emails/text messages and phone calls. They can construct a model of everyone you interact with, and everyone they interact with, and when. They can (and do) record public conversations as well perform video surveillance. The power they have is to link everything together, put a name to a face, a real name to a forum post, and do it all very fast.
Remember the story about the guy who put the cnn logo on the wrestlers face trump hit with a chair? Finding that guy was childs play compared to what they can do to link people to anonymous accounts. As long as you post in both public/private and anonymous social media venues, you can be matched up to your real self. Everyone has a writing style. Phrases they use more often than others. Mis spellings or grammar idioms. You might post something innocuous like you went to an amusement park. They can match millions of occurrences and index them. You might be one in thousands of people who match an individual search, but when they cross index those searches you will quickly be the only one who matches multiples.
Most of this capability has been separated from domestic police activities in the past, but now we have an administration that embraces it. There is evidence they are beginning to use this data against critics. And eventually, they will use it against groups.
