This post from Bang The Table was particularly good, from my perspective, because of the advice in the following paragraphs. Regardless of what platform you’re using, you should probably give some decent thought to the kinds of demographic information you need to know to make your data legitimate — or might need to know in the future. Read the BTT post for the full thing if you’re interested.
When you set up a new site, put time aside to really think about what kind of demographic information you require. Suburb, age and gender are more common ones, but there are also registration forms that ask for participants’ general interests, their household set up and even income range. The more data you collect during the registration, the more you will be able to segment your online community later and find out what part(s) of your community you might need to target more to build a diverse audience.
While it is tempting to ask for as much information as you can think of, the signup form should not be so long or intrusive that it creates a barrier to participation. To avoid this, consider making questions optional and telling your prospective participants why you collect the data and what you are going to do with it.
If you already have an existing site and you feel like it is too late to start asking some demographic question, be aware that your participants at any time can update their profile. You might just have to show them where…