1. A method of managing contact groups, performed at a server having one or more processors and memory, the method comprising: identifying a set of saved groups, wherein each saved group has been created by a user and consists of a set of unique identifiers of people; receiving email header information for a plurality of electronic messages that were sent during a designated period of time, wherein the email header information for an electronic message includes a unique identifier of the sender, and a unique identifier for each recipient of the electronic message; building a set of de facto groups using the received email header information, wherein each de facto group consists of a set containing the unique identifiers corresponding to the sender and recipients of an electronic message for which email header information was received; for each respective de facto group, computing a respective usage value as a sum of respective weights for electronic messages corresponding to the respective de facto group, wherein each respective weight is based on how recently the respective message was sent, and an electronic message corresponds to a de facto group when the set of unique identifiers corresponding to the sender and recipients of the electronic message is equal to the set of unique identifiers in the de facto group; storing each de facto group and corresponding usage value in the memory; comparing the de facto groups to the saved groups, wherein the comparing includes: comparing membership of the de facto groups to membership of the saved groups; assigning a respective usage value to each saved group using the usage values of the de facto groups, wherein a respective usage value of a respective saved group is set to 0 when membership of the respective saved group does not equal membership of any de facto group, and when membership of the respective saved group equals membership of a respective de facto group, the usage value of the respective saved group is set to the usage value of the respective de facto group; and, for at least one de facto group that is different from any saved group in terms of membership, comparing the at least one de facto group and its associated usage value to the saved groups and their associated usage values; and providing a recommendation to modify the set of saved groups based on the comparisons.