Is a general theory of memory possible? What contribution can sociology make to this vast scientific project? Two original contributions are presented and discussed in this article: (1) the concept of collective memory (Maurice Halbwachs); (2) the concept of social memory (Niklas Luhmann). The author proposes some important theoretical clarifications. First, memory is neither a stock nor a collection (of past states or events) but rather a continuous operation of sorting between remembering and forgetting. Secondly, collective memory is not social memory: indeed, the former is a specific operation of psychic systems whereas the latter is an operation of communication, specific to social systems. In the particular case of the attacks of November 13, 2015 in Paris, the author shows the function of social memory that the mass media system fulfills and how these operations of filtering meaning condition the construction of traumatic memories.