Where'd you go? Most mid-size/large festivals are kind of...well...I'm not a fan, to put it mildly, because of exactly how you described it.
Delaware, and yes, it was weirdly, dehumanizingly large, with large hoards of people forming massive rivers as they migrated from one stage to another. The entire weekend I kept catching glances of people at their top / bottom, dancing like crazy or bruised and doing drugs out of dollar bills on the lawn, only to never see them again. But it was still fascinating to see the people who went all out to every apparent outward extreme to have fun: dressing up, making signs, dancing, and drunkenly enjoying the shit out of their food. It was both a turn-off and weirdly attractive to see how happy people were when they let go for the weekend. I do feel bad for the service workers who had to clean it all up afterwards though.
Firefly! A coworker was there for five or six days. He slept for 30 hours afterwards. I saw him last night, and he tells me, "Yea it was a ton of fun. The four of us got a night's worth of sleep every three days. We spent like $600 on drugs. Each." Don't feel too bad for the service workers. I have a ton of friends who work festivals, and not only do they get paid pretty fat day-rates, they often get time off to see shows.
I imagine a combination of ecstasy/MDMA, long-term and short-term psychedelics, and sundry stimulants. Because all regular life-responsibilities are abdicated for a few days, those festivals approach a level of experimentation usually unheard of.