jquery serialize and multi select dropdown

I have been using the jquery serialize() function to serialize the values of a form and submit it via ajax

like for e.g. if the form name and id is factoryUsers

var data=$("#factoryUsers").serialize();

Now this works fine for forms that have text fields, text areas, simple drop downs etc. But when I have a multiple dropdown , things go awry for e.g. if I have a dropdown of the type

<select size="5" id="factoryUsers" name="factoryUsers" multiple="multiple">

the serialize doesn't work correctly anymore. so if I select 3 users I get a query string like

factoryUsers=5&factoryUsers=23&factoryUsers=11

changing the select to array type doesn't help either factoryUsers[]

Any idea or help how to get this working correctly would be great.