Most open source projects still use mailing lists, which are even better than forums in that everyone can read and write emails in whatever client they prefer.
And yes, it's far better than chat for serious discussions, with a global group of people. You don't want to exclude the Americans from important decisions when everyone in Europe is awake and on the chat in the morning, and vice versa.
I think both systems have their places - if I have a quick question, chat is usually better. For longer more serious discussions, perhaps with code included, email wins.