See my comment above. If you assume that the underlying value of Facebook hasn't changed in the last week (a reasonable assumption - remember, we're dealing with the underlying value of the company, not the value of the stock on any given day), then there would be no reason for a bump to occur on the first day of trading except that the stock was incorrectly priced.

The reason that a 'bump' occurs is that money is being left at the table - ideally, stocks should exhibit no bump, because that means that the company has priced their stock correctly. Any bump means that traders are reaping the profit of the increase in the stock's value, and that profit is what the company was seeking to gain by going public.