jpch - 10:52 am on Sep 16, 2011 (gmt 0)
Taking the point farmboy makes above about X+Y one step further I'm willing to bet it's going to be more like X+Y+Y+Y+Y if you add a service like AddThis below each post.
What you probably can't tell now is how many people forward that newsletter to friends that never subscribe. You'll see that reflected with increased page views though. Also, be sure to place a newsletter signup box in a good location on each page.