One big upside is that many employees can start cashing out, and the newfound wealth of a successful Facebook IPO would be widespread enough that it should be easy to spot.
People work at tech startups work for mediocre salaries and stock options that they hope someday will turn into BMWs and houses. That path hasn't been so clear at Facebook, however, because the company has given out different types of non-cash compensation at different times and changed its rules across the years.