The Google Prediction API, as currently implemented, is great for classifying data into a discrete set of categories, however, as noted in the documentation:
Avoid having a high ratio of categories to training data in categorical models.
Try to have at least a few dozen examples for each category, minimum.
For really good predictions, a few hundred examples per category is
recommended.
The Prediction API's classification doesn't work well when the ratio of categories to examples is high and in the example you sketched the relationship is one-to-one because you are trying to find the user whose liked product list is most similar to the user of interest (to find a set of promising products to recommend). In this model, each user is a unique category.