When you upload a photo or video, you'll have the option to turn sharing on or off for each of the social networks (like Facebook or Twitter) you want to share to. You can also share existing posts in the same manner.
To learn more, see Sharing to Social Networks.
To connect and start sharing your Instagram posts to another social network:
Toggle the social network you wish to link to and a login window will appear.
To disconnect and stop sharing your Instagram posts to another social network:
If you're trying to relink Instagram to a social network, you may have to visit that site's settings to unlink your previous account.
Facebook: See your app settings
Twitter: See your authorized apps
Tumblr: See your Preferences and revoke access from the Applications section
To change where your posts are shared on Facebook:
When viewing the Upload window, first choose your photo or video to upload, then click the
Sharing button, and select the Facebook Page (or Timeline) from the Facebook dropdown.