Before it was the world's most popular messaging app, WhatsApp wasn't even a messaging app. Founder Jan Koum simply thought it would be neat to open his address book and see status messages—at the gym, in a meeting—next to everyone's names. He also knew no one wants to endure the rigamarole of creating a username and password, maintaining a buddy list, and joining yet another social network just to know what their friends are up to. So Koum let people log into WhatsApp using only a phone number. He also used the iPhone's Address Book API to automatically scan your contacts to see who you knew who was already using the service.

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Great growth-hacking, yes, but helping people find friends proved more of a side effect than anything. "I was just lazy and couldn't remember my Skype password," Koum says. "I kept having to get new usernames and start all over. I went through, like, three different accounts in the matter of a summer, and I was like, 'Screw this.'" Looking back, though, he considers the decision central to WhatsApp's massive success. "You look at the painful experience you'd have with some of the legacy messaging apps on a desktop, and the elegance and simplicity of SMS," Koum says. "To us, it was just like, well, if SMS can do it, why shouldn't we?"

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.

But Who Are You?

Virtually every powerful company has tried to assert itself as The One True ID. Facebook Connect followed you around the web, making your Facebook credentials a virtual passport to other sites and services. Twitter always hoped to turn your @username into a similarly powerful login, and your profile page into your personal website. The +YourName convention of Google Plus might have turned into something similar, if only anybody used Google Plus.

Beyond the tech giants, the popular open source tool OpenID united your many emails, screen names, and profiles with a simple URL. The Fido Alliance brought Google, Visa, Samsung, Intel, and others together to create a powerful, secure login device you could use anywhere. The Obama administration even got in the game, developing the controversial National Strategy for Trusted Identities in Cyberspace. Under that system, every user would have a "single credential," like a card or a piece of software, that could be used to log into any website or platform. Some of these programs made real headway, but none could get universal support.

But a phone number? Everyone has one. The universality of smartphones turned the address book into a gold mine for anyone building a social app. Facebook Messenger's explosive growth happened largely because you only needed a phone number to sign up. When Google created the Duo and Allo messaging apps, the company opted not to associate your profile with your Google account but with your phone number. "Your contact list is fully populated with lots of your friends' phone numbers," Nick Fox, Google's VP of messaging, said at the time. "You don't need to manage new contacts. Whatever contacts are in your phone work within the app." It also helped that you could send someone a message in Allo, and if they weren't using the app they'd get your message as a plain ol' SMS—with a nudge to sign up.

As more of your personal life moves online, having a single way to identify yourself matters. It helps you find people, helps people find you, and helps keeps you safe. And while people change email addresses when they switch jobs or tire of being fartman420@hotmail.com, a phone number has remarkable staying power. Now that you can port your number between phones, plans, and even carriers, you have no reason to change yours. And odds are your phone's area code indicates where you were living when you first got a cell phone—like a badge of honor, a statement of personality wrapped up in three numbers.

Can I Have Your Number?

Your phone number provides far greater security than a password, but it isn't perfect. Scammers can steal your identity using only your digits, and spoofing someone's number or even stealing it right off a SIM card remains shockingly easy. There are other problems too: You've probably never told Amazon your Wells Fargo password, but they both know your phone number. "Your cell phone number is … tied to the same portals of information that are aligned with your social security number," private investigator Thomas Martin wrote earlier this year. "The little known secret is the cell phone number is more useful because it is connected to hundreds of databases not affiliated with your social security number."

So be careful who has number. You may even want to pick up a burner, or use a service like Sideline and Burner that provides disposable phone numbers. "Dating and Craigslist were the two primary use cases at the beginning," says Greg Cohn, Burner's cofounder. Over time, Cohn says, the Burner crew was amazed at "how many different use cases people had for phone numbers, and extra phone numbers." Customers include celebrity users who don't want their primary number getting out, and people in sales looking to separate work from personal calls on a single device.

Eventually, your phone itself could replace your number as your primary identity—at least when it comes to authentication. Some apps don't require a password at all, but text you a code each time you log in. Smart home devices are programmed to spot your phone, and assume it's never more than a few feet from its owner. You are your phone, and your phone is you. The trend will only accelerate as wearables become more popular and you start strapping stuff to your body instead of shoving it in your pocket. You'll unlock your phone with your face, pay for stuff with your thumbprint, and log into Facebook with your voice.

For now, though, there's nothing more personal than your phone number. So hang onto it, treasure it, be choosy about who you give it to and what you type it into. Those digits represent you more than any username or email address or password. So next time you give someone your number, make sure they know just how honored they should feel.