Those techniques described here were pioneered by Facebook and Google. Basically they are (automatic) CAPTCHA. I still remember the news articles refering a pa

Those techniques described here were pioneered by Facebook and Google. Basically they are (automatic) CAPTCHA. I still remember the news articles refering a patent Facebook filed about audio fingerprinting as the evidence of Facebook eavesdropping on the people. Also technologically with some encryption argorithms, a company can claim that they are not capable to identify user with the fingerprint. But in theory given enough data they still can.