How does Facebook chose which "became friend with" stays on your timeline ?

[Pinch]

I noticed that on Facebook, on your timeline, some "became friend with..." stay on your wall even after a while, like it's an important event. I wonder how FB determine this ? For example i had a romantic story with a girl recently and i noticed the "... became friend with..." is still on my timeline (and hers) 3 month after. Thing is, there's no way for FB to know that we are so close, we did not have a lot of public FB activity between each other and there's not lots of pictures of us together. I'm wondering if they check the personnal messages... I noticed that the few other "became friend with" on my timeline are very close friends, but oddly there's 2 or 3 people i'm not that much close with...
When i say "I'm wondering if they check the personnal messages", i'm also thinking about the number of personnal messages (or in the chat). I don't think they would check what's in the messages.

[Daniel]

I do not think FB would do that and invade your privacy. If anything I could think it's just a coincidence that the FB system does that to you. Or maybe it could be because you haven't done much FB activity with "other" people.

I hoped this helped!