Opening social media links in a new window
First of all, make sure you declare your variables using var:


  assigning a value to an undeclared variable implicitly declares it as
  a global variable (it is now a property of the global object)


Personally, I'd probably add a class to the links in question like shareLink then you could simply do:

$(".shareLink") to select all three links

Alternately, you could try selecting by id ending in _share assuming that quality is unique to these three links:

$("[id$='_share']")

I'd also suggest chaining so that your whole code would be something like this:

$("[id$='_share']").click(function(){
    var w = parseInt((screen.width - 600)/2)
    ,   h = parseInt((screen.height - 400)/2)
    ,   cwin = window.open($(this).attr('href'), 'closewin',
               'status=0,toolbar=0,location=0,menubar=0,directories=0,
               resizable=0,scrollbars=0,height=400,width=600');
    cwin.moveTo(w,h);
    return false;
  }).attr('title','opens in a new pop-up');


Finally, I'd review the documentation on Window.open to be sure you don't want to provide left or top parameters, instead of immediately moving the window, or even a different solution. Make sure you at least read the section on Usability issues