Looks like Facebook have realised its cheaper to make custom switches than use mainstream products.
While this fall is bad, at some point Microsoft/Azure are going to come to the same conclusion and Arista will lose even more annual revenue. The infrastructure and power costs of using off the shelf hardware is likely to be >50% higher than running your own hardware based on AWS and Google presentations. Those savings likely represent 2%-3% of a cloud providers running costs based on approximate costs (i.e. https://perspectives.mvdirona.com/2010/09/overall-data-center-costs/#comment-711044).
TL;DR: In 2018, Facebook/Azure spent >$400m/year on Arista. But not for long.