Starbucks is one of the best marketers going, and look how viral an email can be for starbucks! http://www.marketingvox.com/archives/2006/08/31/starbucks_email_promo_gets_out_of_hand/
Since viral marketing compels people to actively engage your marketing message rather than passively hear or view it .. I'd say ... mission accomplished! Especially since the people who began spreading the message about the free drink were friends and family. It's like giving a personal recommendation and this can be very powerful! Clever Starbucks .. caught up in their own 'virus'.