British Midland International's (BMI) Pinterest lottery
The goals of the campaign were to drive awareness to the newly created Pinterest account for BMI and incentivize users to pin destination photos to BMI's boards (thus creating a large amount of user-generated content). Here's how the campaign was implemented on Pinterest:

Destination boards were created on BMI's Pinterest account. Each board contained exactly nine pins that each had a unique number on them (1-9). Each week, a number would be chosen at random. Everyone who had re-pinned the image that corresponded to that number was entered into the drawing. A winner was chosen at random, and the prize was a round-trip ticket from any BMI destination.

Why this is one of the best social media campaigns of 2012:
BMI used Pinterest exactly the way it is meant to be used -- to encourage the sharing of images. BMI capitalized on the natural action taken by Pinterest users (re-pinning) and created an incentive to re-pin. Travel and destination photos are highly popular on Pinterest, meaning the campaign reached the target demographic of not only BMI but also of Pinterest.
Source: Social Media Examiner