public HttpResponseMessage<lightUserInfo> Post(LogOnModel model)
{
    if (ModelState.IsValid)
    {
            if (Membership.ValidateUser(model.UserName, model.Password))
            {
                MembershipUser user = Membership.GetUser(model.UserName);
                var userinfo = new lightUserInfo(user);
                return new HttpResponseMessage<lightUserInfo>(userinfo);
            }
    }
 }

$.ajax
  url: Meshable.rooturl + "/api/authentication"
  data:   JSON.stringify authenticationDetails
  dataType: "json"
  type: "POST"
  contentType: 'application/json; charset=utf-8'
  error: (e) ->
  success: (data) ->
    #data comes in as empty {}

using System.Runtime.Serialization;

[DataContract]
public class UserInfo
{
    [DataMember]
    public string mystr {get; set;}

}