Microsoft was another popular destination at Stanford at the time. If Facebook was toy-ish and Google was cool, Microsoft was "respected". Many deemed Microsoft to be the McKinsey of tech: spend a couple of years there, build their resume and join or start a start-up. I even knew someone who took a job at Yahoo! over one at Facebook (Needless to say, most everyone who had offers from either Google or Microsoft chose them over Facebook).
Hindsight is 20/20 =)