AJAX CAll Snding JSON Data

Hi all, I don't know where I went wrong but my AJax call is not working. Below is my code:

$.ajax({
                    type: "POST",
                    url: 'http://localhost:12345/MyWebService.asmx/MyMethod',
                    data: JSON.stringify({textToReplace: variable1,  replacementText: variable2,  pageTitle:variable3}),
                    contentType: "Application/json; charset=utf-8",
                    dataType: "json",
                    success: function (response, status) {                       
                        alert(response.d);
                    },
                    error: function (response, status) {
                        alert('Failure!');
                    },
                });

 I have also tried to pass JSON data like below:

 data: "{textToReplace:" + variable1 + ",replacementText:" + variable2 + ",pageTitle:" + variable3 + "}",

Needless to say it too did not work. When viewed in Firebug the error message goes away too fast.

Please point out where my mistake is if you can spot it. Thanks in advance.