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Illustration: Sarah Grillo/Axios

Cellphone numbers have become a primary way for tech companies like Facebook to uniquely identify users and secure accounts, in some ways becoming a proxy for a national ID.

Why it matters: That over-reliance on cellphone numbers ironically makes them a less effective and secure authentication method. And the more valuable the phone number becomes as an identifier, the less willing people will be to share it for communication.

Driving the news: Facebook faced criticism this week for its handling of phone numbers that users provide for the purpose of two-factor authentication (2FA) — in which a person's login is protected by both a password and a device like their smartphone.

  • Some users found Facebook had enabled people to connect their profiles and phone numbers, transforming 2FA from a way to protect personal security into a personal privacy breach.
  • Facebook pointed to a privacy control option that allows users to limit, but not entirely eliminate, this sort of identification.

The big picture: American culture and law are hostile to establishing any sort of national ID, leaving businesses and organizations to find substitutes.

  • Passports don't work for non-citizens, and drivers' licenses are handled by states.
  • Social Security numbers were created to track workers' contributions to retirement benefits but gradually got drafted for other uses by, among others, the IRS and the health care system.
  • Many Americans try to avoid broadcasting SSNs online. But now people have to share them with so many institutions and clerks that there's very little that's truly secret about them.

Background: The internet lacks its own identity system. Email addresses were long a popular but imperfect choice.

  • Each address is unique and, once verified, is useful for receiving information intended only for you.
  • But email is an insecure system, ridden with spam, and once the addresses became essentially free it became easy for one person to use many accounts or quickly switch to new ones.

What's next: Cellphone numbers are becoming Americans' latest quasi-identity system.

  • Once Congress mandated that you could take your phone number from one provider to another, the U.S. ended up with a de facto "cellphone number for life" system.
  • You can switch, of course, but it means changing how your whole social network connects with you.

To be sure: Since so many of us carry a phone at all times and use it as a wallet and a diary, it's natural for it to be treated like a set of keys as well.

  • Some privacy-conscious apps, like Tinder, give users the option of signing up with a phone number rather than through Facebook.
  • That practice only makes people angrier when they feel they're victims of a bait-and-switch like the one Facebook is being charged with.

Be smart: The more people use phone numbers to unlock things, the less widely they'll want to share them, which makes them less useful for connecting people.

Go deeper

In photos: Students evacuated as wildfire burns historic Cape Town buildings

Firefighters try, in vain, to extinguish a fire in the Jagger Library, at the University of Cape Town, after a forest fire came down the foothills of Table Mountain in Cape Town, South Africa, on Sunday. Photo: Rodger Bosch/AFP via Getty Images

A massive wildfire spread from the foothills of Table Mountain to the University of Cape Town Sunday, burning historic South African buildings and forcing the evacuation of 4,000 students, per Times Live.

The big picture: Visitors to the Table Mountain National Park and other nearby attractions were also evacuated and several roads including a major highway, were closed. South Africa's oldest working windmill and the university's Jagger Library, which houses SA antiquities, are among the buildings damaged.

Updated 3 hours ago - Politics & Policy

3 killed, 2 wounded overnight in Kenosha bar shooting

Three people died and two others were hospitalized with serious injuries after a gunman entered bar in Kenosha County, Wisconsin, the police department said in a statement on Sunday.

The latest: Officers arrested a "person of interest" Sunday afternoon in connection with the 12:42 a.m. shooting and there's "no threat to the community at this time," per a later police statement.

Updated 3 hours ago - Sports

Big European soccer teams announce breakaway league

Liverpool's Mohamed Salah (L) after striking the ball during the UEFA Champions League Quarter Final Second Leg match between Liverpool F.C. and Real Madrid at Anfield in Liverpool, England, last Wednesday. Photo: John Powell/Liverpool FC via Getty Images

12 of world soccer's biggest and richest clubs announced Sunday they've formed a breakaway European "Super League" — with clubs Manchester United, Liverpool, Barcelona Real Madrid, Juventus and A.C. Milan among those to sign up.

Why it matters: The prime ministers of the U.K. and Italy are among those to express concern at the move — which marks a massive overhaul of the sport's structure and finances, and it effectively ends the decades-old UEFA Champions League's run as the top tournament for European soccer.