The shift means that new users posting on Facebook will share data only with friends, not the general public, unless they change privacy settings.
The tweak is the latest effort by Facebook to respond to concerns about privacy, following criticism over its data mining practices for marketing purposes.
"While some people want to post to everyone, others have told us that they are more comfortable sharing with a smaller group, like just their friends," Facebook said in a blog post.
"We recognise that it is much worse for someone to accidentally share with everyone when they actually meant to share just with friends, compared with the reverse. So, going forward, when new people join Facebook, the default audience of their first post will be set to friends. Previously, for most people, it was set to public."
Facebook added that first-time users will see a reminder to choose an audience for their first post, and if they don't make a choice, it will be set to friends.
The social network allows its users to change the audience for a post at any time, including of past posts.
Facebook said it will also soon roll out "a new and expanded privacy checkup tool, which will take people through a few steps to review things like who they're posting to, which apps they use, and the privacy of key pieces of information on their profile."
The company said it made the changes in response to feedback from users "that they are sometimes worried about sharing something by accident, or sharing with the wrong audience."
Facebook, which has grown to more than a billion members worldwide, has been dogged by privacy issues for years, as well as by lawsuits claiming it fails to ensure personal data is safeguarded from marketers and third-party apps.
The group agreed with a US government agency to submit to external audits of how well it guards users' data.