First, provided that you have logged in, you may send a request to get your profile using:
AsyncFacebookRunner asyncFacebookRunner = new AsyncFacebookRunner(facebook);
asyncFacebookRunner.request("me", new fbRequestListener());
Then on the listener, you can get username field on the response.
public class fbRequestListener implements RequestListener() {
@Override
public void onComplete(String response, Object state) {
/**Parse the json response here to get the username
Then, append it to the url as "http://www.facebook.com/" +username*/
}
/**don't forget to override other methods*/
}
Hope that helps. :)