Tyler's line: "I'm six-five, 220 pounds, and there's two of me!"
The scene where Christy sets the silk scarf Eduardo just gave her on fire and drops it on his bed. The look on his face when he turns around is priceless.
Christy: When have you even seen me wear a scarf?
Eduardo: This could be the first.
Also the terrified look on his face and the "Oh God please don't kill me" tone of voice as he breaks up with her.
Christy: You're going back there already?
Eduardo: Yes! *beat* And also I'm breaking up with you!
After extinguishing the fire (which, if left unchecked, could have set the entire bed and then the room on fire), Eduardo tells Mark he'll be going to San Francisco, hangs up the phone... and Christy's standing right behind him. She has this pouty, puppy-dog look, as if she hadn't just, y'know, set something on fire.
How about Eduardo protesting that he doesn't know how to change his relationship status on Facebook? Hee!
The whole character of Christy, really. She looked so serious and genuinely worried about Eduardo not changing his relationship status on Facebook, and seemed flatly ignorant of the fact that he had just been played out by Zuckerberg and Parker. Poor Eduardo had come to realise he wasn't quite right about Asian girls being so ideal, but it is played for laughs.
A lot of the stuff with Eduardo and the chicken, which the movie thankfully acknowledges is quite ridiculous and sometimes plays it for laughs despite it still be a rather important/serious part of the story. Some gems:
Eduardo: "I did not torture the chicken! I don't torture chickens!"
Eduardo (defending his actions): "DON'T FISH EAT OTHER FISH?! Th-th-the marlins, and the trouts!?"
And don't miss Joe Mazzello's awesomely deadpan "(pause) What's he talking about?" in response to that line.
Sean (after Mark and Eduardo mention the chickens 5 times in a row): "Seriously, what the hell's the chicken?"
Mark facetiously referring to the Winklevoss twin as the "Winklevii" - apparently, Mark Zuckerberg studied Latin in real life.
The guys using a zipline on the roof for diving into the pool...and promptly taking out half of the chimney due to weight.
This encounter between a girl who keeps getting the same link in her e-mail and her friend.
KC: Seven different people spammed me the same link.
Tyler: This idea is potentially worth millions of dollars.
Summers: You might just be letting your imaginations run away with you.
Tyler: Sir, I honestly don't think you're in any position to make that call
Summers: I was the US Treasury Secretary. I'm in some position to make that call.
And definitely the reaction of the brothers at the end, in response to to Summer's dismissal and his secretary previously berated them while sitting in chairs about the age of the building:
"Whoops. Broke a three-hundred thirty-five year old doorknob." (tosses doorknob onto desk before leaving)
A student completely missing the fact that the guy who'd just spent hours giving a presentation was Bill Gates, even when he said the next Bill Gates could be in the room. Listen closely and you'll hear one of his friends threaten to shoot him with a Glock. Made even better by the audible car screech in between "I didn't even know who the speaker was." and "It was Bill Gates".
"Are you medically stupid?"
The Hypocritical Humor in the scene between Sean and Amy; when he pretends he doesn't know her name, she starts to get pissed off, only to be surprised when he introduces himself as Sean Parker, showing that she had no idea what his name was either.
And this little exchange from the same scene:
Amy: You're not like fifteen years old or anything, are you?
Sean: [Chuckling] No. [Concerned] Wait, you're not like fifteen, are you?
"You know, this is where they filmed Towering Inferno." "... That's comforting."
In the beginning when the Facemash business shuts down the Harvard server, Dustin goes to check his computer to confirm what the guys saw on Mark's. For a split second you can see the excitement on his face (if not his body language) as he leaves his spot at Mark's chair, then it becomes obvious a few seconds later. It's pretty amusing in its own simple way.