To link your Facebook and Twitter accounts, open the Instagram app on your phone or tablet, and select the Profile tab in the bottom-right corner of the screen. In the top-right corner of the profile screen, tap the gear icon on the iOS or Windows Phone version of the Instagram app; Android users should press the three vertical dots for the app’s Options menu. On the Linked Accounts screen, select Facebook and follow the instructions.
Repeat the process by selecting Twitter on the Linked Accounts screen and entering your login information.
After you have connected your Facebook, Twitter and any other services to your Instagram account, if you want to post a photo to all (or some) of them select the photo and apply your filters. When you get to the caption screen, tap the Facebook and Twitter buttons to include them in the upload.