All sites should know about their readers. This can easily be done with a demographics survey. Sometimes without a survey you can pull demographics from other sources. Alexa gives you a overview of the visitors on your blog. If you have a Facebook Fan page for your site you can also find some of the demographics there. For me, I set up a survey on Google Documents. It was pretty easy to set up.
I set up the survey to be able to easily identify my audience. By knowing who my readers are, I can produce better articles for my readers. Articles that they would be more interested in.
I made a QR code for my demographics survey.
I appreciate everyone who fills out this survey. And believe me, I would never share any of your information.