One of the safest and simplest computer-security measures available is gradually becoming commonplace. Two-factor authentication adds a layer of protection to the standard password method of online identification. The technique is easy, relatively quick and free. So, what's the problem?

Critics are quick to point out the shortcomings of two-factor authentication: it usually requires a USB token, phone, or other device that's easy to lose; you sacrifice some privacy by having to disclose your telephone number to a third party; and it is subject to "man-in-the-middle" and other browser- and app-based attacks.

Still, as more hacks and database breaches occur, two-factor authentication is the most practical protection available. The number of big-name services supporting two-factor authentication continues to grow: Google, Facebook, Yahoo, PayPal, Twitter, Snapchat, Microsoft, LastPass and Dropbox are among the sites that let you require two-factor authentication to sign in to your account from unverified computers and devices.

Something you know and something you have

If you've used your bank's ATM, you've used two-factor authentication: you insert your ATM card (something you have) and enter your passcode (something you know). Most Web services supporting two-factor authentication send a unique access code to your phone, but banks and other financial services may require a hardware token that either displays a code you enter or that you insert via USB, smart card or other port.

Here are the steps required to activate two-factor authentication on some of the most popular Web services.

Facebook

To activate the service's Login Approvals feature, sign in to your Facebook account. Click the drop-down arrow in the very top-right corner and choose Settings. Select Security in the left pane, then click Edit to the right of Login Approvals. Next, check "Require a security code to access my account from unknown browsers."

A window opens explaining how log-in approvals work. Follow the prompts, which include adding a phone number to your account (if haven't already done so) and entering a confirmation code sent to your number to verify you are, well, you.

You can also take advantage of the code generator feature within Facebook's mobile applications, should you not have cellular signal when attempting to log in to the site. The code generator is found within the app, but sliding out the "More" menu and scrolling down to the Settings section. There you can expect to find a Code Generator option, which will display a six-digit code when launched.