Why you should sharpen your Instagrams
Image source: http://art-pete.com/art/i-am-sitting-in-stagram/
UPDATE (07 JULY 2015)
Instagram has upgraded their app and servers to host higher resolution images now, so it's not THAT important to pre-sharpen your photos before you post them. You can read more about it here:
Regramming the same image 90 times
The above Gif is what happens when you regram the same photo (via screenshots) 90 times on Instagram. Artist Pete Ashton posted an Instagram, screenshot it, regrammed the image as a screenshot and repeated this process 90 times. You can read more about it in detail in his blog post here.
All the processes of file conversion and compression degrade the image quality ever so slightly each time an image is screenshot and regrammed. Instagram also has to compress and convert your image so that it can efficiently distribute your Instagram worldwide, to all phone types and screen sizes (thanks Android).
These server side decisions highlight how Instagram has had to make some hard but necessary choices which ultimately degrade image quality to ensure a better quality of service. Instagram co-founders Kevin Systrom and Mike Krieger actually mention this process in their Re/code interview which you can watch here.
Why you should (slightly) sharpen your Instagrams before you post
If you haven't already noticed, before you even post your images to Instagram, the app itself reduces the sharpness and quality. Check out the two screenshots below (taken from an iPhone 6 Plus). The left image is from before I selected the image, the right image is from after. Click to enlarge.
Before selecting an Image to Instagram
After selecting an Image to Instagram
It is the exact same image. If you look at the bookshelf in the background of the image, you can see that the lines of the shelf and books are slightly blurred in the after shot (on the right). This degradation is actually visible when viewing from my phone's screen. You can test it yourself. Try it on an image with fine lines and detail and it will be noticeable.
This is why you should pre sharpen your images (very slightly) before posting to Instagram. Of course, it might not be necessary for all your Instagram photos as it might suit some photos, or be less noticeable on others. Whatever you do though, don't go overboard with the sharpening as it might actually end up making your photo look worse.