How does Facebook determine the box of 9 friends on Facebook timeline?

[Louise]

A number of factors seem to contribute to the reasons why you will see certain friends photos show up more often in your 9 Friends picture box on your own profile. These factors include:

Interactions on facebook (recently and over time). Recent interactions play a large role, especially if someone has recently viewed your profile page.
Profile views (they’ve viewed your profile and/ or you’ve viewed their profile – Total amount of views when compared to other friends profiles viewed)
Photos that you’ve been tagged in (or have tagged your friends)
Wall posts
Likes
Comments (on your posts or comments you’ve made on your friends wall posts or pictures)
Photos viewed (they’ve viewed your pictures, and/or you’ve viewed theirs)
Online – Friends who are currently Online (or recently been online for Random people that might show up)
Chat – Facebook IM chat messages between each other
Private messages sent between friends
Any “mutual interactions” between yourself and your friends will likely play a bigger role in determining who your “closer” 8 friends may be.
If you’ve added friends to your “Close Friends” group, typically at least 2 of these “Close” friends will be shown in the 9 friends box.