I suspect even had Facebook gotten early funding from someone in Boston, they would've moved out to the Valley (or at least the west coast) for the same reasons many YC companies do. This is mostly a story about Battery losing a potentially good payoff, not a story about Boston losing its one chance to have a young software startup stick around.
It would depend on how much they raised. YC companies are unusually mobile. Most haven't raised enough to hire anyone, and it's employees that tie you down. Few companies move after they have 20 people, unless they get bought. So if FB had raised enough to hire that many people in Boston, odds are they'd still be here.
