If you have actually blocked somebody mistakenly or changed your mind about the block, though, it is also reversible. You can change your Personal privacy settings to unblock and permit someone back into your Facebook world.
How To Unblock Somebody On Facebook
1. Click the down arrow beside "Account" on top right of any page on Facebook.
2. Click "Personal Privacy Preferences" from the Account menu.
3. Click the "Edit Your Lists" link under the Block Lists heading. It is at the bottom center of the page.
4. Click the "Unblock" link beside the name of the individual you wish to unclog. A list of the individuals you've obstructed is found under the Block Users section.
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Uncloging somebody does not immediately put that individual back in your friend list. You should include the freshly unblocked person as a good friend and wait on them to confirm the request.
And one more thing please don’t forget to share this awesome trick to use the How To Unblock Somebody On Facebook with your friends.