The general consensus is that they timed it to coincide with Apple's announcement, so that the news of them laying off employees would be buried by all the Apple press.
I think it has more to do with getting the cuts out of the way before their Q3 earnings report comes out tomorrow. Facebook just mentioned in their earnings call this afternoon that they've been hit by a 20% YoY drop in revenue from Zynga, so you know Zynga's results are going to be bad.