Note: You can also just create a web handler which can be even lighter weight than JSON.  I have used web handlers for things like activation servers and really high traffic servers.  I create my own EDI format and then serialize it back and forth between the client and server, and you can't get any lighter since I am 100% in control of every byte being transmitted.

[WebMethod]
public string MyTestWebMethod()
{
  using(MyBO bo = new MyBO())
  {
     bo.FillWithWhatever();

     //-- Do whatever you need to do

     //-- Return the results
     return bo.MyFullName;
  }
}