If you've installed WhatsApp, then they do. If I hash your number that you installed whatsapp on and get X, and then hash your number in Facebook and get X, an

If you've installed WhatsApp, then they do. If I hash your number that you installed whatsapp on and get X, and then hash your number in Facebook and get X, and then realize that X == X, then I just matched your contact list in WhatsApp with all of your data in FB, all without explicitly sharing your phone number. Even better, I now can match up all your friends, and all of their likes and dislikes, and make some educated guesses about what your likes and dislikes are. Hashing, long story short: put some data in, like a phone number, and get a unique, unguessable strong back -- the same string, every time. https://simple.wikipedia.org/wiki/Cryptographic_hash_function https://simple.wikipedia.org/wiki/Cryptographic_hash_functio...