How can I make Instagram photos shared to Facebook visible to my friends by default?
As Andrew Ferrier stated in the comments to the question, this is indeed a part of Facebook's privacy settings. Here is how you set it to share to "Friends" by default:


Go to your Facebook.com.
Find the page to edit your Instagram settings.

this should be on YourLinkToTimelinePage/app_instapp
or you should be able to find it at https://www.facebook.com/settings?tab=applications
or in the timeline menu under your profile picture, find it in More

Change your App visibility and post audience setting to Friends

This setting controls who on Facebook can see that you use this app. It also allows you to choose the audience for posts the app makes on your behalf.


Hope this helps you.