Besides choult's point, users can often be de-anonymized based on just a few ratings. Someone did this by cross-referencing dates in the Netflix data set and th

Besides choult's point, users can often be de-anonymized based on just a few ratings. Someone did this by cross-referencing dates in the Netflix data set and those available on one of the bigger sites. Suddenly the fact that you were watching documentaries or movies that let you infer their political or sexual proclivities could be determined by outsiders.