What makes share a Coke campaign so special

Coca-Cola is a multinational drink company that is one of the largest multinationals in the world and has one of the most successful marketing campaign and brand. In 2015 Coke released its "share a coke" campaign, it was the best-performing marketing campaigns in Coca-Cola's history. This campaign encouraged customers to buy a bottle of coke with their name on or buy one for their friend. 
This campaign connected with customers on a personal level creating loyal customers for coke.

Coke started the hastag #shareacoke
for it's customers to share pictures online of themselves with their bottles of coke, the success of the campaign online was huge with over 500,000 photos shared with the hashtag #shareacoke. Coke gained nearly 25 million more followers on Facebook due to the campaign.

Reference:
http://www.investopedia.com/articles/markets/100715/what-makes-share-coke-campaign-so-successful.asp