Here’s Matt Cutts on one of his “Grab Bag” videos in which he answers questions from webmasters all over the world. Neil from Newcastle wanted to know if the number of spam complaints need to reach a certain count before Google conducts a review of a website or a search engine results page. It is safe to assume that Google receives thousands of such complaints on a daily basis so it would be interesting to know how these are handled and what determines their order of priority, if any.

Matt confirms that the Webspam Team does get a large volume of complaints. With the finite amount of resources that they have, they are forced to devise methods to sort these in the most logical manner. Their foremost consideration is always the impact on the users. For instance, if the offending site is a popular one that is visited by lots of people then fixing that issue will be prioritized over another site, which is rarely seen by anyone.

Google compiles the results of each case in order to figure out how they can improve the spam filters and other deterrents. Everything they learn gets applied in future iterations of their algorithms.

Video Link:
How are spam reports prioritized? –