Windows Phone Instagram client 6tag has been updated, bringing the version number 4.1.2.
The update brings a fix for Direct messages. An issue that cropped up while selecting friends to message has been corrected. Instagram's Direct feature allows you to send messages with pictures directly to your friends, rather than sharing them privately. You can send Direct messages to as many or as few friends as you choose while in the sharing section of the app.
Thanks to Aman and Ankit for the tips!