As described in 1852:
We find that whole communities suddenly fix their minds upon one object, and go mad in its pursuit; that millions of people become simultaneously impressed with one delusion, and run after it, till their attention is caught by some new folly more captivating than the first.
Men, it has been well said, think in herds; it will be seen that they go mad in herds, while they only recover their senses slowly, and one by one.
Charles Mackay, LL.D. (1852). Memoirs of extraordinary popular delusions and the madness of crowds.
It seems little has changed in the past 165 years. Social media enables the mania to occur more rapidly!
(With thanks to the late John Heffernan who suggested I read this book.)
Just after writing the above, I saw this set of headlines at Reuters.com