You can become an expert in one tiny narrow subject, or you can learn how you became an expert in one topic and apply that ability to learn onto other subjects. For example, so many of these social networking experts were probably excellent face to face networkers who transferred their skill set to the online world. My husband repairs homes for a living. He probably wasn't an expert in everything to begin with, but took an ability to understand how things work in general and applied it to a wide range of topics.
Are you an expert in something? If you aren't, your job, your effectiveness, and your personal brand are in danger. Want to become an expert? Pick up a book, search the net, and listen to other experts every single day. Apply yourself just a little bit at a time, over a long period of time. Have a passion for it and you'll be surprised at how soon others recognize your expertise.