Here's a great graphics cheat sheet for anyone managing multiple social media channels.  Image sizes and shapes are all over the map, from wide to tall, from square to round.  And, this is just for web browser interfaces.  3rd party social media clients such as Tweetbot add yet another twist, and even apps by the same company may look different depending on hardware platform.  Tweetbot, for example, displays profiles in rounded squares on their Mac client while using circles in their iOS app.  

As image makers, we're always considering the composition of our photos and videos.  It's not enough to capture a well lit, in-focus shot.  We need to think about how the image (still or motion) will be used.  Do we leave space for copy (text)?  If so, where?  Above?  Below?  To the left or right?  Social media interfaces add another twist - how can you leverage an image for a square format and an extra wide rectangle at the same time?  More thoughts on that issue coming soon.