In October 2014, Twitter introduced Digits, a way for developers to sign up users with just a phone number. Particularly in developing countries, where some users may not have email addresses, a phone number-based login can spur rapid growth. As an added benefit, it avoids most of the pitfalls associated with password-based logins — it's much harder to steal a phone number than it is a password. Digits is part of Fabric, Twitter's suite of developer tools, which announced last week that it is now running on 2 billion devices. Maybe that's why Facebook copied it. Say hello to Facebook Accounts!

Accounts, which were introduced on stage at today's F8 developer conference, represent a significant expansion of the familiar "log in with Facebook" button. Apps that enable logins with Accounts ask you for your phone number. Once you send it, Facebook sends you a code to confirm the number belongs to you. Enter the code and you're now logged into the app, with no password to remember. "People tell us that they hate passwords," says Eddie O'Neil, a product manager at Facebook.

Traditional Facebook logins are likely still more valuable to developers — they are much richer in data, which is helpful for both advertising and growth purposes. (In the latter case, Facebook logins often include information about who your friends are, which apps then use to suggest you invite them to download it themselves.) But in a test with Indian music app Saavn, new sign-ups increased by 33 percent when Accounts was introduced into the app, Facebook says. Today 85 percent of the top-grossing apps in the United States already use Facebook logins. With Accounts, that percentage is poised to increase — and to blunt any momentum Twitter was gaining with Digits.