Social Media allows you to strike up a conversation with your customers and build a relationship.
Here is a list of some of the most popular social media properties (data from Wikipedia):
Facebook: 500,000,000 users, Founded 2004, General interest
Twitter: 175,000,000 users, Founded 2006, General interest, Micro-blogging, RSS, updates
Habbo: 165,000,000 users, General for teens, over 31 countries world wide, Chat rooms and user profiles.
MySpace: 130,000,000 users, Founded 2003, General interest
Linkedin: 80,000,000 users, Founded 2003, Business and Professional Networking
Facebook and Twitter are the two that most people know about. I find Linkedin useful to connect with industry professionals.
sources: searchengineland.com, wikipedia.org