I have at least one person on my friends list who last signed on June 1, 2007. From that I infer either that: they sweep the old accounts out at irregular times, or that friends stay on your list even if they got deleted, or that the deletion isn't made until the end of the sixth calendar month, or they stopped deleting old accounts.
I'm interested in seeing what happens February 1. If he's still there, maybe the policy has changed. Don't know him in real life, so I don't know what happens if he signs back on now.
I know that banned accounts stay on your friends list even after a ten year ban goes into effect, since Mizbubble still shows up on my list, and she got a pretty wicked ban and hasn't been on since August.