Trends come and go, though the length this one has stayed surprises even me. But when a trend is started by one of the biggies in mobile (Google was the first to use it heavily IIRC), then people tend to follow the trend train.
Personally, I’m kind of surprised that FB is joining the parade after so long. They typically do their own thing and ignore whatever everyone else does.
Some usability person claims that the circle adds more emphasis to faces, but I don’t buy that…