//avem nevoie de ...
var windowWidth   = $(window).width();
var windowHeight  = $(window).height();

var faceboxHeight = $("#facebox").height();
var faceboxWidth  = $("#facebox").width();

//centrarea elementului
$("#facebox").css({
    "position": "absolute",
    "top":  parseInt(windowHeight/2) - parseInt(faceboxHeight/2),
    "left": parseInt(windowWidth/2)  - parseInt(faceboxWidth/2)
});