1. A method, comprising: at a computing system having one or more processors and memory storing programs executed by the one or more processors: accessing event records associated with multiple users of a website, each event record identifying a respective user activity at the website and having an associated event type; generating a respective event transition matrix for each of the multiple users, each respective event transition matrix being generated in accordance with a respective user's event records; wherein each element of each respective event transition matrix includes a probability of an occurrence of two consecutive event types A and B for the respective user; wherein at least one element of each respective event transition matrix corresponds to two distinct event types; wherein event type A has occurred at least once during a predefined time period; and wherein the probability of the occurrence of the two consecutive event types A and B for a respective user is, at least in part, dependent on a count of the occurrence of the two consecutive event types A and B during the predefined time period divided by a count of the occurrence of the event type A during the predefined time period; categorizing the multiple users into at least two distinct groups of users based on an analysis of the multiple users' event transition matrixes; and performing one or more operations to user accounts of the website, the user accounts being associated with one or more users from one of the at least two distinct groups of users.