How to enable 2FA for Facebook?
Install Secure ID
Secure ID enables you to have a single mobile app for all your 2FA accounts and you can sync them across multiple devices. Install Secure ID on your device by searching for it in your device’s app store .
Locate 2FA Setting in Facebook Web app
Are you using Facebook mobile app? 
1. Login to Facebook and click down arrow displayed at top right hand side. Select the option Settings.
2. Click Security and Login displayed on left hand side.
3. Scroll down a little on facebook page. Under Two factor Authentication click Edit button displayed next to Use two-factor authentication.
4. Click Get Started button and after selecting Authentication App option click Next button.
Two Factor Authentication screen with QR code is displayed.
5. Launch Secure ID app on your device. Click “+” icon displayed and scan the QR code displayed on Facebook’s Two-Factor Authentication screen .
6. Click Next button on Facebook Two-Factor Authentication screen.
After scanning the QR code Facebook logo is displayed and six-digit security code is generated by Secure ID app.
7. Enter the six-digit code(generated by Secure ID) on facebook page.
8. Click Finish button on the facebook page.
Congratulations! Secure ID is now ready to use for Two Factor Authentication for Facebook.