Great followup. Two answers:
1. The DOJ does get their info from the cellphone companies. They also suck it directly off the internet backbone and store it away in their data centers for later assessment/decryption. (Except the NSA's new data center that was supposed to last then 20 years was full before it opened.)
2. The DOJ has to follow rules to gather your data. Facebook doesn't. Every one of those, "What kind of potato are you?" quizzes on Facebook are created by people who want access to your data. By taking the quiz, you allow that app/company access to enormous amounts of data about you, your friends, your activities, your locations, your likes, your dislikes, etc. The quiz itself is a simple Eliza script that is totally meaningless. The point of these quizzes is not the quiz. It's the permission; you give that company access to your personal data and use metrics and network, in perpetuity. There is a lot, they can find out, about you and your life.
Now, take every place you have ever been (Check-Ins, photo metadata, friend's tagging you, GPS location tracking), and correlate that with every single phone call, email, fax, voice message, text message, ATM transaction, and Google Search you have ever done...
... and the idea of "Precrime" from Minority Report looks like child's play. (And doesn't need any mystically-mind-linked flesh goobers to make it work, either.)