Just received a notice to respond to the 2011 Population Census. Normally, we'd fill out a paper form and just put it in the mail. In our case, however, we have to respond online because we were chosen to fill out the "long form" questionnaire. The government randomly chooses 10 percent of the households to answer more detailed questions about themselves, and this form is available only online.
If you prefer, there are also census workers who will come to your home to conduct an interview. A worker will also visit you if you fail to respond.
I can't remember if it's mandatory to participate in the U.S. Census, but here, we'll be fined HKD 500 if we fail to respond, and HKD 5,000 if we give false information. I know what I'll be doing this holiday weekend....