The capture everything done online and store it. They can wait until you become a person of interest then plug your email address and name into the system to get your Amazon, eBay purchases, Google searches, FB comments and likes, GPS data from your phone to see where you went, who you were with by cross referencing FB friends and their phones GPS data.
They are not looking at you specifically but they are keeping your data in case one day they do want to look at you.