No points, just clarifying
For scalability of your form, leakim's solution is the best as it wraps up everything in the form to send to the server in nice name value pairs.
Gary's on the other hand has shown you that you need to use the val() function on each of your parameters to get it's value.
I was going to say that it could be the web method requires certain formatting of parameters but that would time out Gary's solution, which you know works. See what the article can give you re jQuery Ajax and web methods
Thinking about this some more, this isn't about sending form data to a server as such but it is about calling a WebMethod, which requires certain parameters.
You're wanting to call a WebMethod called SendContactInformation that has the parameters fullname, phonenumber, emailaddress and reason.
Therefore you should only be JSON encoding these elements of the form.
It's not smart enough to only take the required parameters from the serialized data.
In another scenario where you might be submitting a form to a server side script for processing you may serialize it but the server script will parse the serialized data into its constituants for processing. Very differnt from calling a method.
This shows how form data is sent and processed on the server side.
Microsoft also have their own JavaScript framework that I suggest you look at rather than jQuery. It may cause compatibility issues and the event model works slightly different.
I always use the ASP.Net method of postback and calling the database, etc. I don't want to say the "old" way but since I'm learning jQuery and another way of coding, I have to call it the "old" way
In this article you'll learn how to use Ajax calls within your CodeIgniter application. To explain this, I'll illustrate how to implement a simple contact form to allow visitors to send you an email through your web site.
The viewer will learn the basics of jQuery, including how to invoke it on a web page.
Reference your jQuery libraries: (CODE)
Include your new external js/jQuery file: (CODE)
Write your first lines of code to setup your site for jQuery.: (CODE)