Jeffrey Zeldman:

Finally—and here’s the part that freaks some people out—your friends’ faces will show up on websites where they’ve clicked the “like” button. Think about that. You’re on Joe’s website. You see your wife’s, girlfriend’s, and minister’s faces smiling at you from Joe’s website. The people who matter to you, and who you thought you had compartmentalized in the privacy of Facebook, a non-public-facing, password-protected website, are now out in the open. (Of course, they are out in the open to you. Achmed will see his friends, not yours. Still.)