Some folks suspect it's people who view your profile most often. Others suggest it's the people you view most often.
I looked at mine and I beg to disagree with the latter group. Honestly, I hardly view the profiles of the folks who appear in my sidebar. So does that mean the former group is the right one? I don't know. And I'm not about to ask them, "hey, do you check my profile often?"
It's one mystery that I'm perfectly fine not knowing the answer to.