Andy Samberg surprised the crowd at f8 by posing as Mark Zuckerberg. He made a few jokes and basically parroted some lines from his Saturday Night Live skits. Just after announcing a fake new feature called "slow poke," which is like a real poke except for a "you get two days to withdraw," the real Zuckerberg walked out, Samberg asked him for a billion dollars and Zuckerberg asked if it was for weed again. As Mediagazer's Megan McCarthy pointed out, the whole stunt is a looks a bit like Noah Wyle standing in for Steve Jobs at MacWorld 1999. Zuckerberg standing on stage, sweating a little already, made it immediately clear that he has a long ways to go to reach Jobsian charisma.
Samberg is definitely less of a look-a-like.