It's a dark-blue app with a white "f" on it. Doing so will open your News Feed if you're logged into Facebook. If you aren't logged in, first enter your email address (or phone number) and password. It's either in the bottom-right corner of the screen (iPhone) or in the top-right corner of the screen (Android). You'll find this option toward the bottom of the menu. Skip this step for Android. It's either at the top of a pop-up menu (iPhone) or toward the bottom of the ☰ menu (Android). This tab is near the top of the screen. This option is near the bottom of the screen. It's the second option near the top of the page. Doing so will prevent anyone who doesn't belong to any of your friends' friend lists from randomly adding you on Facebook.

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Write a summary of the previous text in Vietnamese:
Mở Facebook. Chạm ☰. Kéo xuống và chạm Settings (Cài đặt). Chạm vào Account Settings (Cài đặt tài khoản). Chạm vào Privacy (Riêng tư). Chạm vào Who can send you friend requests? (Ai có thể gửi lời mời kết bạn? Chạm vào Friends of friends.