For example, Intelius does background checks, reverse phone number lookups etc. One small piece of information can lead to others. As far as unwitting, of course. The wikipedia page says that company has acquired background check companies, a facebook genealogy app company, classmates.com and a "people" search engine company... not to mention all the public databases that are available (criminal records, property records) The've been collecting online and offline data on people for almost 20 years. I'm sure they have data scraped from sites that don't exist any more. Combine that with the current terminally online population and I think it would be surprising to most regular people how much data is out there.