I don’t tend to like design fads, but despite all the naysaying on Twitter, I don’t think circular avatars are terrible. Here’s why:
They are helpful differentiators. Most information on the web is presented in boxes. By placing avatars in circles, it sets them apart from raw information and delineates social/personal content from everything else.
They are naturalistic. Faces are more round than rectangular. Using a shape that complements the natural construction of the face is, in many situations, more harmonious than using a standard box.
Just because they’re a fad doesn’t make them valueless. Yes, plenty of designers use circular avatars because they’re “in” right now. That doesn’t mean they don’t have appropriate use cases.