In the past brands would contact a media buyer, pay them a ton of money and get their ads onto the channels which had been selected. Social media has changed all of that. Now brands pay consumers directly to share their campaign message via their personal channels.
Great idea from Starbucks is the Tweet-a-coffee idea. People in the US link their Starbucks and Twitter accounts and then using the @tweetacoffee handle, send $5 Starbucks gift cards to each other.
Simple and clever.