All a "writedown" means is a reduction in the recognised value of an acquired asset or company.

In this case Zynga states:

"...$85 million and $95 million (excluding any income tax impact) related to the intangible assets previously acquired in connection with the company's purchase of OMGPOP."

So Zynga is saying that they overpaid, or that the asset is not performing as they wished, i.e. so they are writting-off the intangible value (the difference in price between OMGPop's real assets when acquired and its acquired price). In this case they are writing-down $85m-$95m of a reported $200m dollar acquisition - a 43% to 47% reduction in value.