When you upload high resolution media on a social networking website or send it to a friend on an instant messenger, the applications reduce the quality of the media to a lower resolution in order to save storage space on servers. Facebook is no stranger to this tactic.
However, if you want to upload a full resolution image on Facebook, there is a way to work around this. According to Facebook, images that are at least 2048 pixels wide are not affected, apparently. When uploading pictures on Facebook, it is helpful to select “Create Photos Album” and check off the “High Quality” box before uploading, and you’re all set.
Instagram also now allows users to upload images larger than 640 X 640 pixels.
Here is an interesting comparison of the media upload sizes on different social media websites like Twitter, Tumblr, Pinterest, Facebook, LinkedIn, Google+, etc.