For an individual, the set of data those websites have on who matches you is around "Your total living relatives out to 6th cousin or so" * a ratio of people wi

For an individual, the set of data those websites have on who matches you is around "Your total living relatives out to 6th cousin or so" * a ratio of people willing to use the site's services. That turns out to be a lot of people. And when you correlate that with data on where people live and where they lived at the time of a string of crimes, it turns out to be a lot of data.