I downloaded a copy of my Facebook data last week. I didn't expect to see much. But when I opened my file, it was like opening Pandora's box. I learned that about 500 advertisers - many that I had never heard of, like Bad Dad, a motorcycle parts store, and Space Jesus, an electronica band - had my contact information, which could include my email address, phone number and full name. Facebook also had my entire phone book, including the number to ring my apartment buzzer. The social network had even kept a permanent record of the roughly 100 people I had deleted from my friends list over the last 14 years. Facebook said unfamiliar advertisers might appear [in the file] because they might have obtained my contact information from elsewhere, compiled it into a list of people they wanted to target and uploaded that list into Facebook. Brands can obtain your information [by] buying ... from a data provider like Acxiom, which has amassed one of the world's largest commercial databases on consumers. Let's be clear: Facebook is just the tip of the iceberg when it comes to what information tech companies have collected on me. Knowing this, I also downloaded copies of my Google data with a tool called Google Takeout. The data sets were exponentially larger than my Facebook data. Here was the biggest surprise: In a folder labeled Ads, Google kept a history of many news articles I had read. Be warned: Once you see the vast amount of data that has been collected about you, you won't be able to unsee it.