There are three simple steps in between you and the clarity of your Facebook account. There is a short script involved, which you will copy and paste into the console of the Developer tools of your browser (Firefox, Chrome) while navigating into the Facebook Friends page.
In Short, what you will be doing:
Get the script. Go to the Facebook Friends page. Run the script within the browser.
Copy in your clipboard the code
Go to the Facebook Friends page: https://www.facebook.com/YOUR-ID/friends Scroll all the way down until the last friend is shown in the thumbnail.
Press F12 to open the Developer tools window (Firefox, Chrome). Select Console. Read the security concerns message that the Facebook developers left for you. Paste the script into the prompt of the console Type pleaseUnfriendAllPlease(), hit return, and wait until it’s done.