I define Hareidi as what it means in its most common usage: anybody and everybody who wears a fedora. Hence, I think the Hareidi movement began in the 1880/90s witht the advent of the fedora, eventually grew to include almost all of the western world, and drastically shrunk in the late 1900s to include just a couple of Ultra-Orthodox Jews, mostly in New York and Israel.
As a matter of fact, every time I take off my hat after Mincha, there is one less Hareidi in the statistics.