WhatsApp was among the first apps to equate your account with your phone number. Now apps like Snapchat, Twitter, and Facebook Messenger do it too. Starting this fall, setting up your iPhone will be as easy as punching in your number. The supposedly super-secure way of logging into apps involves texting you a secret code to verify your identity. Phone numbers are killing the username, killing the password, and making it easier than ever to go wild online. So guard it with your life, because it is your life.
Source: How Your Phone Number Became the Only Username That Matters | WIRED