You've probably seen the message from Facebook about expanding it's facial recognition features.
You may love the idea. You may hate the idea. But if you're looking to turn it off...
From the mobile Facebook app
Launch the Facebook app, go to your profile page, and tap the More button all the way to the right beneath your cover and profile photos. Select View Privacy Shortcuts from the popup and scroll down to More Settings, and tap Face Recognition. Here, it'll ask you whether you want Facebook to be able to recognize you in photos and videos, tap the question and select No if you want to disable the face recognition feature altogether.
From your desktop browser
Log onto Facebook, tap the down arrow all the way to the right on the menu bar and select Settings. Tap Face Recognition, and on the subsequent screen click Edit on the right. Select No from the drop-down to disable face recognition.