I haven't seen Social Network, so I can't comment on that, but prior to SN, how was this false? I'd have to strongly disagree based on the roles I've seen each play. In fact, when I first saw Zombieland I actually forgot that Eisenberg WASN'T Cera until about halfway through the film.
Just taking Zombieland and Juno and comparing Eisenberg and Cera, I think that they have the basic similarity of being kind of awkward, but other than that they're very different. Cera's character - granted I haven't seen Scott Pilgrim, but I have seen Juno/Arrested Dev/Superbad - is of a very introspective, inward, loner kind of awkward. His characters have serious lack of confidence (in ArrDev, worries about manhood too) and don't really work socially because they have no idea what to do.
Eisenberg's most Cera-ish character is in Adventureland. Even there, though, he's a little sharper, a little more cynical, more talkative. In Zombieland, though, he's very different: he's a survivor, obviously, kind of sees life as a video game. He's like a teenager who's become a bitter jaded old man already. "Don't be a hero." Social Network. Cera, like as the runner guy in Juno, is the kind of awkward who really wishes that some girl would let him be the hero.
I guess what I'm getting at is that the Cera stereotype is really innocent and sweetly foolish, and that's what makes him awkward, while the Eisenberg stereotype has a really exaggerated sense of what he wants and needs, and can't relate to other people, and that's what makes him awkward.
I've actually been meaning to write about Eisenberg/Cera for a long time. Now that I have a blog, maybe I should do an essay on them.
