It's whatever you want it to be. You're responsible for the impact you make, not how long you're at your desk. Facebook has high impact expectations, but if you're smart, you can have a big impact and still have a life.

Facebook culture also has a big social aspect. I think I made more personal friends at Facebook than I made in the entire rest of my career. One downside (according to some people) of this effect is that Facebook tends to become your social life. Personally, I appreciated the personal relationship formation, since it's rare to see so many smart people in one place.