Pretty much 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.