Facebook has removed something from the source code of profile pages (the OrderedFriendsListInitialData array of numbers) and, unfortunately, this tool no longer works.
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The actual formula for how Facebook prioritizes your friends isn't known. We can assume however that they have a prioritized list of our friends somewhere and that it's a combination of the social actions you make (eg who you message most often, who's profiles you visit and for how long, who's status/images you like, etc). A friend showed me what he thought was this list as it's found in the source code of everyone's logged in profile page. Here's how to find the list and an easy way to see who the people are.
Built by @jcbmllgn
Start by going to your Facebook profile page, right click anywhere, then click "view page source":
This show's you the html source of the page. Now push command + F and search the page for "OrderedFriendsListInitialData".
You should see something like this:
Now highlight and copy the whole array excluding the first and last quotation mark.
Paste all of those numbers here, push go, then scroll down to see the list.
I am not storing any information about you or your friends, feel free to view the source code for this page to see exactly how it works...
This may take a minute to load. See the list below.
No, I'm not 100% sure that this is the actual prioritized list, however, based off the results for me personally and the dozen or so people that I've show this to, it seems like it could be.
Feel free to shoot me an email with your thoughts.