Budget writers had estimated that Facebook stock would sell at $35 per share when the company sold a large batch of stock in November.
Instead, the share price dipped below $25.
"The Facebook hit was a fairly sizeable hit for the budget."
Jason Sisney with the Legislative Analyst's Office says it amounted to a $626-million decline in expected revenue. But he says forecasters couldn't have ignored what appeared to be a likely tax windfall.
"Given the certainty that Facebook was going to generate a lot of money, I don't think it was a viable idea to just put in zero, but as it turned out the stock performance was not nearly as robust as many people projected and that resulted in less revenue."
Sisney says unexpected personal income tax revenue is making up for some of the loss.