The only alliance I've been in that used chat was Hogs of destiny, and you always had about 10 players on at the same time because of the size of the alliance, which got to 2000+ players.
I've been in alliances that have tried to use it but in the end Skype is always the go to chat. The in game chat has too many issues when compared to most external ones.
I find in-game chat just gets in the way all the time hense why I use skype, it's quick doesn't get in the way of grepo and it's much more advanced than in-game chat for obvious reasons