here's the easiest way to explain it:
let's say SA has about 1,800 people who want to play wow.
1000 of these people roll horde on 'ganis, 150 sounds about right for the original size of Hooligan, 50 go to other guilds, and the rest join Goon Squad.
500 of these people roll alliance on kel'thuzad, most of whom join The Goon Squad
300 or so go play on random servers somewhere.
things start friendly between hooligans and GS, people level, I /friend darkidol, and things are happy. Then someone gets their feelings hurt and ruins relations between EJ (right when they changed to EJ) and GS. GS undergoes a series of splinters, but still maintains around 800 members. Everyone kinda ignores The Goon Squad on Kel.
so, in open and stress test beta everyone was either on server #4 or server # 29 (?). at release we split up to Kel'Thuzad and Mal'Ganis.
note: numbers are made up to give a general idea, not specifics.