Step 1:
To see any friend’s hidden friends list, you need to download and install the Facebook Friend Mapper extension.
Step 2:
Now go to your friend’s profile whom you are wishing to see the hidden friends list. You will be seeing a new (Reveal Friends) option in the friends tab.
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Step3:
Click on the Reveal Friends option and you will be able to see all the friends whether they are your mutual friends or not.
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Warning: These kinds of extension are not good for your web browser, I just installed them in order to make the tutorial and write about this flaw. So I won’t recommend you to use this on a permanent basis.
If someone uses this tool, your privacy settings won’t work for that person and your hidden friends list will be exposed. I hope Facebook fixes this logical flaw before this tool goes viral.
You can install and check out the Facebook Friends Mapper Chrome extension for free.