Stopping a visual in an infinite scrolling stream takes place under a second. The commonality of prominent shares is surprisingly simple: a clear focus, a strong contrast, and a message read at a glance.
The crowd loses visuals. The more a frame tries to tell, the less it tells. The best social media designs magnify a single idea; leaving the rest to the work of the text or the next frame.
There is also the fact that 'mobile first': when designing your visual, it is necessary to assume that it is monitored on a small screen, with low attention and quiet. Design running on a small one works everywhere.