Just when the Instagram is being touted as The vehicle for building brand recognition, Starbucks has found a way to make bucks by way of Twitter. It's pretty clever and not something that comes straight from Twitter cookbooks for marketeers. Based on the buzz being generated, it is safe to assume that other retailers will borrow from the idea.
This is a lot like giving a gift without having wrapped it so everyone can see. In the "real world" that would be a social faux pas, but somehow this is not. The chosen few (or even one) of someone's followers could be tweeted a coffee because they are so awesome. Those that are left behind will not feel quite so awesome. Back in the day when that gift may have come via email, no one would have been wiser. The expectation here is some customers will act of out of impulse to be cool and trend setting which will prompt herd behavior among those they are connected with. It will not matter so much that this is at odds with established etiquette for gift giving - the coolness will be enough to mitigate that.