20. A conversation server system, comprising: one or more physical processors; memory; and one or more programs, wherein the one or more programs are stored in the memory and configured to be executed by the one or more processors, the one or more programs including instructions for: storing a plurality of index entries in an index, a respective index entry corresponding to a respective term and having a plurality of index components, a respective index component of the respective index entry identifying a message that is associated with the respective term; receiving a first message; associating the first message with a conversation having at least one other message; and storing, in the index, a plurality of first-message index components that each include an identifier of the first message, including: one or more index components indicative of a plurality of message terms in the first message; and one or more index components indicative of one or more conversation terms in the conversation, the one or more conversation terms comprising one or more terms that are not in the first message; wherein a respective first-message index component indicative of a conversation term in the conversation includes a value identifying the respective index component as being associated with a conversation term not in the first message.
23. The system of claim 20 , including instructions for, after storing, in the index, the one or more first-message index components indicative of the one or more conversation terms in the conversation: detecting a modification of a respective message of the one or more other messages in the conversation; and in response to detecting the modification, updating the one or more first-message index components indicative of the conversation terms that are stored in the index in accordance with the modification of the respective message.