If you want to consume a webservice, it's not allways necessary to do a postback.
It's even not that hard!

1. Webservice

You have to add the scriptservice attribute to the webservice.

[System.Web.Script.Services.ScriptService]
public class PersonsInCompany : System.Web.Services.WebService { 

Create a WebMethod

    [WebMethod]
    public Person GetPersonByFirstName(string name)
    {
        List<Person> personSelect = persons.Where(p => p.FirstName.ToLower().StartsWith(name.ToLower())).ToList();
        if (personSelect.Count > 0)
            return personSelect.First();
        else
            return null;
    }

2. webpage

Add reference to your service to your scriptmanager

<script type="text/javascript">
      function GetPersonInCompany() {
          var val = document.getElementById("MainContent_TextBoxPersonName");
          PersonsInCompany.GetPersonByFirstName(val.value, FinishCallback);
      }

      function FinishCallback(result) {
          document.getElementById("MainContent_LabelFirstName").innerHTML = result.FirstName;
          document.getElementById("MainContent_LabelName").innerHTML = result.Name;
          document.getElementById("MainContent_LabelAge").innerHTML = result.Age;
          document.getElementById("MainContent_LabelCompany").innerHTML = result.Company;
      }
  </script> 

Add some javascript, where you first call your webservice.
Classname.Webmethod = PersonsInCompany.GetPersonByFirstName
Add a callback to catch the result from the webservice. And use the result to update your page.

<script type="text/javascript">
      function GetPersonInCompany() {
          var val = document.getElementById("MainContent_TextBoxPersonName");
          PersonsInCompany.GetPersonByFirstName(val.value, FinishCallback);
      }

      function FinishCallback(result) {
          document.getElementById("MainContent_LabelFirstName").innerHTML = result.FirstName;
          document.getElementById("MainContent_LabelName").innerHTML = result.Name;
          document.getElementById("MainContent_LabelAge").innerHTML = result.Age;
          document.getElementById("MainContent_LabelCompany").innerHTML = result.Company;
      }
  </script>

 
If you have any question, feel free to contact me!

You can download the code here.