How does facebook determine which friends appear on your profile?

[Kendog]

If you just want a short, what I have to say, go down to the paragraph that starts with Basically. for a full explanation to my reasoning, just start VhereV
I've heard many things: that it's completely randomly picked, that it's who visits your profile most, and even who you interact with the most. I, however, am believing it's a little bit formula-less, if you will. The reason for this is because there are parts that do agree with that and parts that do not. I'm just gonna sort of list why I think each does and does not work.

Random:
Plus-There are definitely people on my top 10 friends who have no reason to either visit my profile, nor do we communicate at all showing that there is a bit of random involved.
Negative- There are people on my 10 friends who have complete reason to visit my profile, AND we communicate regularly, showing not all of the formula relies randomness, and an actual formula somewhere is involved somehow.

Who visits the most:
Plus- Well, honestly, this might be more of a hope than anything else. Every time I've been in the stage of talking with a girl, they end up popping up on my top 10 friends eventually, and, no, I do not creep on them, so it's not who I visit the most. On top of this though, there are a couple people who do make sense to visit my profile often. People I don't congregate with, but would make sense to visit my profile DO show up, showing that some people could be who visit my profile.
Negative- Again, there are people who show up there that make absolute NO sense showing up there if it were to rely completely on who visited my profile the most, showing that some formula is required past just who visits the most.

Who you interact with the most:
Plus: There are people who I interact with that show up there that don't make complete sense for stalking on my profile, mainly because they are those Facebook people who we post on each other's statuses often, but we very rarely visit each others walls, showing that this is also a very probable reasoning as to how facebook does it.
Negative: I don't exactly post on facebook too much, I post my own statuses, and see what's up with everybody else in the world, so when I see the same 14 people switching in and out of my 10 on the left side, I have to know that it can't be just this (I typically post on about 3-4 different people's statuses, so the other 7 don't really make too much sense)

Basically, I think that it's a mixture of all three of the most common theories, like 3 are a set random, 4 are who visit your profile the most, and 3 are who you interact with the most, because they all have their strong reasonings, but they all have strong drawbacks as well.