If you have a Starbucks card you can link it to your Twitter account and very simply send a friend a voucher for $5 credit, by Tweeting @ them. Full details in the video below, and on this page as advertised by this Tweet:
Now sharing the gift of @Starbucks is as easy as sending a Tweet. http://t.co/UNbkcjRX1s
— Tweetacoffee (@Tweetacoffee) October 28, 2013
I think this is Twitter's first foray into payment, and I expect there to be many more, given founder Jack Dorsey's obvious skills and connections in this area. He owns Square, who has launched Square Cash, which lets you transfer money simply by emailing. It's pretty much the same thing, but letting you do it by Twitter will make it seem more immediate.
Another thing: Starbucks accounts for 90% of US mobile payments, according to this report.