You have a new product called Microsoft Surface, and decide that part of the advertising budget will go to celebrities seeding good things about it. Afterall, you are going to fight with the iPad, and you need all the help you can get. The more people talk it up, the better it will be.
Then you hire Oprah, and tell her “Post something positive on Twitter about the Microsoft Surface”. And that’s exactly what she does. She opens her Twitter, posts that she’s loooving the Surface, and that she’s already bought twelve units to give away as Christmas gifts. Oh, yes. She posts, naturally, from her iPad.