In order to arrive at economies of scale, you have to first provide for the expenses of ramp-up. So far, admission is nominal for the value its payers perceive. Plotting growth against current cost should give a pretty good forecast of how much is too much. Of course, under the current special circumstances, we could even see a freeze.
Besides the Christmas timing, Yahoo quite recently upgraded the Delicious servers. Apart from whatever capital expense was involved, the occupation of the Delicious team in implementing the migration might have left them with the impression that they had a future. It's impossible to know in a corporation who knows what and who's blowing smoke for the company. Still, the tenor of pre-layoff comments from the team in their unusually candid support forum suggests that at least some of them were totally blindsided.
