Sending String Data to MVC Controller using jQuery $.ajax() and $.post()

There's got to be something I'm missing. I've tried using $.ajax() and $.post() to send a string to my ASP.NET MVC Controller, and while the Controller is being reached, the string is null when it gets there. So here is the post method I tried:

$.post("/Journal/SaveEntry", JSONstring);

And here is the ajax method I tried:

$.ajax({
    url: "/Journal/SaveEntry",
    type: "POST",
    data: JSONstring
});

Here is my Controller:

public void SaveEntry(string data)
{
    string somethingElse = data;
}

For background, I serialized a JSON object using JSON.stringify(), and this has been successful. I'm trying to send it to my Controller to Deserialize() it. But as I said, the string is arriving as null each time. Any ideas?

Thanks very much.

UPDATE: It was answered that my problem was that I was not using a key/value pair as a parameter to $.post(). So I tried this, but the string still arrived at the Controller as null:

$.post("/Journal/SaveEntry", { "jsonData": JSONstring });