Instagram updated their app today to allow users to post full-size landscape and portrait photos. As a photographer, all I can say is, "Finally!!" As long as you have Instagram 7.5 or above, when selecting an image from your photo library you'll notice a little circle format button on the bottom left corner of your image. Pressing it toggles between square and full-size images.
You will of course have the occasional few that will cry out about this announcement, but the majority of my feed is photographers who use apps like InstaSize to make their portrait/landscape photos fit into a square. According to Instagram, around 20% of uploaded photos were modified with 3rd party apps to fit in the standard square format.
Check out Instagram's blog post about the updated app.