SAN FRANCISCO — Do you know who can see what you are posting on Facebook, including your photos, birthday and personal cellphone number?

Chances are that you don’t.

Facebook is worried that you will start sharing less — or maybe even move to more anonymous services — unless it helps you better manage your private information. On Thursday, the company announced that it would give a privacy checkup to every one of its 1.28 billion users worldwide, conducted by a cartoon dinosaur.

Facebook, which is based in Menlo Park, Calif., will also change how it treats new users by initially setting their posts to be seen only by friends. Previously, those posts were accessible to anyone. And it will explain to both current and new users that setting their privacy to “public” means that anyone can see their photos and posts.

The change in default settings and the person-by-person review is a sharp reversal for Facebook, whose privacy settings are famously complicated. Some users may be shocked when they see just how widely their personal information has been shared.