$3.5 million is simply too large a number for your theory to be correct. A number that large to a single employee doesn't come out of thin air, it's based on something -- whether that be formal analysis or back of the envelope calculation.
I suspect the calculation was (expected value of FB package) * 1.5
It's also possible that it was based on the potential damage done to Google by Facebook's talent acquisition, or if this isn't a zero-sum game, to prevent any gains by Facebook.