Fortune writes on what makes Facebook different:
The typical user can see less than half a percent of Facebook’s total user profiles, Zuckerberg says. That is in stark contrast to MySpace, where everybody can basically see everybody else if they know where to look.
Unlike most other social networks, Facebook has sophisticated privacy settings. You have a lot of control over who can see what information about you. So you can enable members of your school or work group to learn about you, but prevent anyone in a high school group from seeing anything, if you choose. You can also determine how information about you shows up in searches other members conduct on the site.
Photo albums are an important part of peoples’ personal profiles, for instance, but the control Facebook offers is so “granular,” to use Zuckerberg’s term, that you can allow one album to be seen by your college mates, another only by work colleagues and bar your siblings from seeing any photos, if you choose. Users are so confident that their information is sequestered from intruders that more than a third post their cellphone numbers.