The first "rule" of social media, if there really are any rules in such an infant industry, is to listen first. Because the social web is more centered around conversation than one-way messaging, you can't just jump in with your marketing messages and blah-blah-blah your way to success. You have to listen first.
Think of it this way: You walk into a social networking event. You don't walk up to a small circle of people and start screaming, "We've got great deals going on down at Joe's Pawn Shop!" If you did, the circle of people would gradually walk away from you, or even have someone remove you from the room.
Instead, you stand close, but not within the circle, until someone notices that you're there, widens the circle, and introduces himself or herself (or allows you the opportunity to introduce yourself). You listen to the conversation until there's an appropriate time for you to interject your thoughts or opinions; when you do, you don't talk about Joe's Pawn Shop. You talk about the subject matter that the group is discussing.
Listening is key in both circumstances. As a person at a networking event, you have to listen to know the topic, other people's biases, and the tone of the conversation. If you don't hear those things, you can't possibly interject something useful. In online communications as a brand, you have to know what your audience is interested in, asking about, or discussing before you dive into the conversation. Without that intelligence, you seem like a spammy company clumsily butting its nose into other people's chats.