A、Politician. B、Executives. C、Teachers. D、College students. D文中提到Facebook的目标用户原本只是大学生，但现在这个用户群扩大了。由此可以推断Facebook最初的目标用户是大学生。

(12)Facebook has half a billion users. That’s huge for a planet of less than 7 billion people. In fact, if Facebook were a country, it would be the third most populous nation after China and India.
    What grew as the pet project of Mark Zucker-berg, when he was a student at Harvard, has exploded gaining eight new users every second for the past 15 months. Facebook has changed the way many people communicate—letting its millions of users share personal thoughts and images with their network of friends.
    Zuckerberg, Facebook’s CEO, said, "People have really gotten connected, not only sharing more information and information of different kinds but being more open and with more people. And the social norm is just something that’s changed over time. "
     (13)People share a lot. Whole lives play out on Facebook: births, graduations, marriage and even divorce. People share personal information on the site 70 times per month on average.(14)And what was once an online destination only for college students has expanded, says Michael Nelson. He worked at IBM and currently teaches at Georgetown University. "It’s executives: it’s politicians. It’s everybody. The fastest growing group of users—it’s the over sixty groups." he said.
    Facebook is so popular that public figures have taken notice. President Barack Obama is credited with using the power of Facebook during his 2008 campaign. Even Pope Benedict has a profile—sharing updates in eight languages.
    Information can easily go viral with the click of a mouse,says Nelson.(15)"Now we have this technology for this one community to talk to another community, and all these different conversations happen at the same time. Almost effortlessly," he said.
Questions 12 to 15 are based on the conversation you have just heard.
12. How many users does Facebook have?
13. What do people share in Facebook?
14. Who are the original target users for Facebook?
15. What’s Michael Nelson’s opinion towards Face-book?