The Facebook app and its web version are very comparable, and both use the same security settings. On the mobile app, perform the following steps for
How to Make a Post Shareable on the Facebook App.
On your Facebook main screen, click the "What's on your mind?" box.
To change the audience settings, tap the "Privacy" link with the globe image below your profile name.
Select "Public."
When finished, continue on with your message.
To post the message to your wall, select "Post" in the top right corner of the screen.