My favorite way of putting it:

If you're walking into a business or public venue or something, and there was someone at the door who, as the cost of entry, asked you your name, birthday, street address, phone number, likes, dislikes, names of all relatives and friends, and all of the places you've recently visited....most people would feel incredibly uncomfortable, turn around, and walk away.

Now imagine it's not a person or a venue, it's the Facebook sign up page. Why should you feel any less uncomfortable just because Mark Zuckerberg isn't standing next to you asking you these things directly?