The entirety of Mark and Eduardo's friendship, which is crowned by this moment: Eduardo flies all the way to Palo Alto to make amends with Mark and continue working on Facebook, arriving at the latter's house soaking wet from the rain. Awww.
A much earlier moment: When Mark declares that (The) Facebook is ready to go live, he shows Eduardo the masthead. Then this:
Eduardo: "'Eduardo Saverin: Co-founder and CFO.' ... You have no idea what that's gonna mean to my father."
Mark: "Sure I do."
Which makes it heartbreaking to see where their friendship goes by the end of the film.
After the Winklevoss twins lose the boat race by a very close margin, their father tells them not to be so hard on themselves. They did their best, and that's what matters.
"No, don't apologize to me after losing a race like that. Don't ever apologize to ANYONE after losing a race like THAT."
In a way, Marilyn's "I don't think you're an asshole, Mark. You're just trying really hard to be" comment is this. Despite everything that happened, there is still someone who thinks there is still one shred of humanity within Mark.