It's been quite sometime since I asked this question. Now I understand it more clearly, I'm going to put a more complete answer to help others.
In Web API, it's very simple to remember how parameter binding is happening.
POST simple types, Web API tries to bind it from the URL if you POST complex type, Web API tries to bind it from the body of
the request (this uses a media-type formatter).
If you want to bind a complex type from the URL, you'll use [FromUri] in your action parameter. The limitation of this is down to how long your data going to be and if it exceeds the url character limit.
public IHttpActionResult Put([FromUri] ViewModel data) { ... }
If you want to bind a simple type from the request body, you'll use [FromBody] in your action parameter.
public IHttpActionResult Put([FromBody] string name) { ... }
as a side note, say you are making a PUT request (just a string) to update something. If you decide not to append it to the URL and pass as a complex type with just one property in the model, then the data parameter in jQuery ajax will look something like below. The object you pass to data parameter has only one property with empty property name.
var myName = 'ABC';
$.ajax({url:.., data: {'': myName}});
and your web api action will look something like below.
public IHttpActionResult Put([FromBody] string name){ ... }
This asp.net page explains it all. http://www.asp.net/web-api/overview/formats-and-model-binding/parameter-binding-in-aspnet-web-api