There was the ep where Officer Barbrady learned to read, and read Atlas Shrugged, and found it so dumb he resolved never to read another book. That’s a good poke at Randroids, if not at Libertarians-in-general. Stone & Parker probably would class Objectivists as an especially annoying sect of Know-It-All College Hippies.
Nigerian 409 scammers?
Interestingly, they also mocked George Clooney (and his Oscar speech) in that one; Clooney was their great champion when they were just getting started, sending copies of their early cartoon of Mr. Hankey, the Christmas Poo around to his friends and generally helping to get the ball rolling.
Does anybody know if they had a falling-out, or are they still friends?
Re. the as-yet-unmocked, I haven’t watched SP regularly for awhile, but have they done any of these?
extreme-sports stars
rock critics (a subset of hipsters)
any big-name music producer
“Morning Zoo”-type rock-station rock/shock DJ/comedy teams
internet memes per se; people who became briefly famous as 'net memes
college football, any college football team (the comedy potential is enormous, when you factor in the coach, alumni booster clubs, mascot, student body, campus stadium, etc.)
“As Seen On TV” products & infomercial hucksters
Vladimir Putin
Nicolas Sarkozy
Hugo Chavez
the persecuted homosexuals of Uganda (what if South Park were to offer them all humanitarian refugee status?)
the now-rescued 33 Chilean miners
yoga, Bikram yoga; spin classes; other exercise fads (big inflatable ball; step aerobics, etc.)
NBC’s Brian Williams
Harvey Levin & the staff at TMZ
Twitter addiction; celebs on Twitter
Lolz addiction
extreme hot sauces/the whole hot-sauce machismo thing; also, the subculture of serious BBQers
the Japanese “do 300 anal sphincter constriction exercises daily to ward off depression” author
the Russian Parkour dog “Tret” (now on Reddit; he’s amazing!)
the Chinese lunar-landing program
Richard Branson’s Virgin aerospace-tourism project
space elevators
the internationally-supported CERN atom smasher that straddles the French-Swiss border
Muse (I love them but there’s a ton of things SP could celebrate and/or mock these guys for, Matt Bellamy especially)
the Indian yogi who claims to never eat anything
tofu (a SP parent could get obsessed over eating and serving his/her family lots of tofu, but it contains phytoestrogens, so a SP take on it could be that the women in the family feel great and lose a few pounds of pudge, but the men and boys get man-boobs – I dunno if this would ever happen IRL, but it makes for a great joke. This could be combined with a celebrity-chef spoof; a celeb-chef visits South Park or shoots a show there, inspiring one family or even the whole town to go wild over tofu; man-boobs ensue! And for extra yuks, have the men lose their sex-drive, even as the women get a boost in their libido…)
I think they did Internet memes, sort of–well, they did “famous” Youtube vids.
They’ve done Facebook but not Twitter.
They did sort of do “As Seen on TV” with Billy Mays and Chipotlaway.
Yeah, they’re still friends. In fact, Clooney has stated that he would be offended if they didn’t make fun of him from time to time.
Bob Saget has also enjoyed being mocked on South Park, I know.
That’s great! (And Clooney’s even cooler for it.)
What does Kim Jong-il say?
One of them backfired. They had a show with Gary Condit and OJ Simpson and the Ramseys and they implied that those people were all guilty and covering it up. Well Condit had nothing to do with Chandra Levy’s murder and it’s kind of ruins the whole episode’s effect.
So mocking things that later on may not pan out isn’t always a good idea
E.g., their several Al Gore/Global Warming eps. 
The Ramseys were also cleared via DNA evidence.
This is like looking in a mirror and saying three times, “Candyman, Candyman, Dio please hijack my thread.”
You’re braver than me.
Well, as Stan said, “You see, I learned something today. ****************************************************************************************************************************************************************************************************************************************.”
I haven’t watched every episode. Have they mocked Trey Parker and Matt Stone?
Actually, they kinda have. From Cartoon Wars part II:
The trucker’s obviously referring to South Park.