Twitter, Facebook and LinkedIn
Twitter's login verification can be turned on the Settings page on the Twitter website. You may need to add your mobile-phone number; if so, scroll down to Mobile, then enter your number. Twitter will text a numerical code to your phone; enter that into the field on your web browser.
Once that's done, select Account in the left-hand navbar and click Login verification. Enter your Twitter password when prompted, then click Send code.
You'll receive another texted code. Enter it into the form field. From now on, you'll be prompted to verify your password every time you sign onto Twitter.
This doesn't apply to third-party Twitter clients such as TweetDeck, which use different methods of accessing your Twitter account.)
Facebook calls its two-factor verification system "Login Approvals." To access it, click on the little gear icon on the top right of your Facebook page, and scroll down to "Account Settings."
On the following page, click Security in the left-hand navigation bar. Click on "Login Approvals" on the resulting menu, and check the box next to "Require a security code to access my account from unknown browsers."
Facebook will text you a one-time code when you log in from each new device or browser.
Like Google, Facebook also offers a second option for smartphone users — a code generator that will create valid one-time codes for when you can't receive text messages (such as when flying).
To turn on Facebook's Code Generator, click "Enable Code Generator in the Facebook app on Android or iOS" from the Security tab in the Account Settings page mentioned above, then open the Facebook app on your smartphone or tablet and tap Code Generator near the bottom of the menu to activate.
Alternately, you can access Code Generator directly from the mobile app and simply activate it there.
Enter the code displayed on your mobile device into the browser to complete the setup. Codes change every 30 seconds.
For branded — i.e., corporate — accounts, Facebook makes users log in from their personal accounts, creating an intermediate step and solving the problem of requiring multiple users' access to the same device. (Twitter has yet to address this problem.)
To enable LinkedIn's two-step verification, users of the business-networking service should click on their headshot in the upper right corner of their profile page. Scroll down to "Privacy and Settings" and click "Review."
On the following page, scroll down to the bottom and click the "Account" tab. Click "Manage security settings."
From there, you'll be taken to a page entitled "Security Settings," with a section called "Two-step verification for sign-in." Click "Turn on."
On the next page, you'll be asked to provide a mobile phone number. Do so, then click "Send code."
You'll receive a six-digit code in a text message. Enter the code into the required field in your Web browser.
As with the other services, you'll have to do this for every new device from which you access LinkedIn.