Sephora's "15 Days of Beauty Thrills" (Facebook)
The goal of this campaign was to engage fans and drive traffic to the newly designed Sephora.com website. Here's how the campaign was implemented on Facebook:

Sephora leveraged some of its beauty partners (Urban Decay, Pantone Universe, Make Up For Ever, etc.) to supply "thrills" for the Sephora Facebook community. These thrills included a $5,000 shopping spree at Sephora, a Fiat decked out with a Gucci interior, a trip to Paris, and more.
Sephora also launched a fan-gated application (meaning that in order to access the application, a user had to "like" the Sephora Facebook page). Fans were able to learn about the new thrills at midnight the night before they launched, and they were able to enter to win these thrills every day on Sephora.com.
In addition, Sephora launched a massive Facebook ad program that targeted its demographic (women between 18 and 35) to drive traffic to the application. Along with the "big thrills," Sephora also kept fans engaged and visiting the website by offering "mini thrills" (e.g., a free eyeliner) with a purchase at Sephora.com.

Why this is one of the best social media campaigns of 2012:
Sephora created a sense of exclusivity by offering insights and special deals to its Facebook fans that they couldn't get anywhere else. The brand offered highly compelling incentives for participation (who wouldn't want to win a trip for four to Paris?) and capitalized on the interests of the target audience. Furthermore, the campaign leveraged partners that could help amplify the message of the campaign, and the entire experience was social; users could share their experiences through Facebook to create a viral effect.
Source: Mashable