I'm going to guess that the fact that Mark Zuckerberg showed up is a sign to assure him that he's going to be a very well-respected and well-tended to employee

I'm going to guess that the fact that Mark Zuckerberg showed up is a sign to assure him that he's going to be a very well-respected and well-tended to employee at Facebook with access to a bigger engineering team, a huge user base, all the resources he wants, and maybe even a chance to get a crack at Facebook software itself. That's pretty enticing.