FYI, for those Americans complaining about the census being mandatory: it's a requirement imposed in the original draft of the Constitution. Specifically, Article 1, Section 2:

Representatives and direct Taxes shall be apportioned among the several States which may be included within this Union, according to their respective Numbers, which shall be determined by adding to the whole Number of free Persons, including those bound to Service for a Term of Years, and excluding Indians not taxed, three fifths of all other Persons. The actual Enumeration shall be made within three Years after the first Meeting of the Congress of the United States, and within every subsequent Term of ten Years, in such Manner as they shall by Law direct.
The census is required every 10 years, because otherwise there would be no way to determine how many seats each State should get in the House of Representatives. That's why you get fined if you don't respond.

Of course, most information beyond name, date of birth, and place of residence is not necessary for this purpose, so the contents of the current census go beyond the bounds of the Constitutional requirement. Just remember that a basic, minimum census of some kind is required in order for our governmental system to work.