I've written about the importance of using user-segmentation to deliver unique user / site experiences and email marketing. Here's a brilliant example of Zynga's Word With Friends (the mega-popular Facebook & mobile Scrabble game).

Obviously the game is predicated on multiple users playing... and that obviously starts with an invitation process. Words With Friends users your Facebook / Twitter networks to create an address book of friends playing the game. The first people in your address book are new users (with big, prominent NEW badges next to their name). It's a super simple, basic concept - but it's brilliant because Zynga knows that:

- new users need to be prompted to play - once they play a couple games, they are hooked - current users feel a sense of goodwill / obligation to play with newly joined friends - current users likely play within the same confined network... this broadens that

Small UI placement that makes an important difference.