About 12 million Americans keep blogs, according to a survey released last July by the Pew Internet and American Life Project. Even more people might blog if the technology weren’t so public. After all, who wants to share a high-school-reunion video with stockbrokers in Istanbul or teenagers in Tokyo?

Privacy controls that let a blog’s author decide who can view each post are a major feature of several new blogging platforms. Vox, a free Web-based service launched by the San Francisco blogging-software maker Six Apart, allows users to assign various privacy settings to each post. The software is free, but bloggers have to accept that an advertisement will follow each of their posts.

Six Apart was already making blog-publishing software when, in 2005, it acquired ­LiveJournal, which has one of the fiercest followings in the blogosphere, thanks partly to privacy settings that are now part of Vox. “Sometimes you only want your five best friends in the world to see a post, and you should be able to do that,” writes Six Apart cofounder and president Mena Trott in her own blog. The Pew survey suggests that this desire is widespread: only 27 percent of U.S. bloggers told researchers that a major reason they blog is to change the way other people think. A larger group, 37 percent, cited staying in touch with family and friends.