PSA: Facebook Wants Your Junk To Be Publicly Searchable.
As the title suggests, all public posts are externally searchable now.
Macworld has the rundown and here is how to change your privacy settings:
But you can change this easily. Log into Facebook. Click the lock in the top menu to the right of the search bar. Avoid the privacy shortcuts and go straight to “see more settings.” Under Privacy Settings and Tools, it says, “Who can see my stuff?” Choose the option to “limit the audience for posts you’ve shared with friends of friends or public,” then select “limit past posts.” Facebook will ask you again if you want to limit old posts, so click again to confirm. It’s a slog, but it will make every single old post viewable to only your friends, so it’s worth the extra steps.