Hey there,

You site user normally can set it from their user profile, you can take a look of my screenshot below.

Or if you would like to global set all the custom field default privacy, you can set it from backend > Easysocial > profile type > select your profile type > privacy tab > profile field section

After you set it, remember tick that checkbox before save (Tick the checkbox to reset all user's default privacy under this profile. ( This will also reset previous privacy settings of items belong to the users.))

Hope this help.