ask for "css ads fb positioning"

hey i wanna ask, how to make css like ads-sidebar on fb,

when you scroll down, the ads follow you,
but when you scroll to the top of the page, the ads not follow you to the top because of some other div (if you are user of fb, you should know what I mean,)

i think this how to use fixed in relative css-positioning, but i'm failed. its not work,

This is when you go to the top of the page (fb)

And this is when you scrolldown

can somebody help me