If you pick up Metro: Last Light promptly upon its March release rather than waiting for a free copy from Facebook, you'll be rewarded with some extra stuff. Launch copies of the game will be packaged as a "Limited Edition," with bonus Ranger Mode DLC and a "Ranger" loadout packed in. In the previous game, Ranger Mode meant less ammo and more damage.
Starting December 18, you'll be able to pre-order the LE on PS3 through the PlayStation Network – which really calls its limitedness into question – and get a free copy of Homefront and a Metro PSN theme. That's Metro: Last Light, not "Metro" the Microsoft UI, of course.