It seems like no matter what I try, the Facebook page plugin always goes on top of my fixed position top nav bar. I have used every combo I can think of. I have set the Nav bar to a very high z-index (99,999 or higher) while setting the z-index of the plugin container very low. But it still goes over the nav bar when scrolling down.
Here is an example page so you can see what I mean, the Facebook plugin is in the right column.
http://www.tinker-mods.com/gallery/xbox-360/custom-made-xbox-360-console.php
Instead of setting z-index to iframe set z-index: 0 to .fb-page div itself and set higher z-index to #myTopnav.