I've just been remembering all the good times on Halo 2, and it got me thinking to before Xbox 360 came out.
In the first years of Halo 2, Halo 2 WAS Xbox Live. Many people don't seem to remember, that there were no parties just in general on Xbox Live, it was a party on Halo 2. Messages weren't sent from the dashboard, they were sent in Halo 2. Much of the aspects used today were adopted from Halo 2, such as Parties, many friends list features, the message system, ETC.
When people asked if you wanted to boost, it wasn't 'what game', it was 'which playlist'. When people wanted to play customs, they instinctively knew they were talking about Halo 2, it was second nature. People used Halo 2 to communicate, relax in parties, play some games, and just 'hang out' if you will. Clans were joined, left, and made for different reasons, and they really helped to bring people together.
I just remember that there were literally no other games, aside from the occasional sports 1v1 game, were played until Battlefield 2 came out, and then it was basically Halo 2 and Battlefield. It just amazes me to think about how we were basically playing Halo Live, anything outside Halo 2 was foreign to most people because no other games could even compete with Halo 2.