How much privacy does one share with his housemates/spouse/siblings? Pretty much nothing. Facebook is one big house where everyone gets to know about each other's activities. Though such sharing might be unacceptable to certain people, most people (especially college under graduates and younger) love it. So this is probably the future!

At the same time, it must be remembered that facebook never forces anyone to share their personal information. You do have control over who sees what.