I subscribed to 4 newsletters which are StackOverflow, Christianity, Beer and Meta SE one week ago, but I haven't received these newsletters yet. Why?
To prevent abuse, one must first confirm his/her email address in order to have the newsletters really be sent.
When subscribing, a confirmation email is being sent automatically, with such subject:
Email Verification - [site name]
Which contains in the body a link "Click here to verify your email address", and once clicked you will start getting the newsletters for that site.
In case the confirmation email was lost, you can resend it by going to your network profile subscriptions tab, and in there you'll see a "resend confirmation" link: