18. A system comprising: a computer-readable memory storing executable instructions; and one or more hardware computing devices in communication with the computer-readable memory, the one or more hardware computing devices programmed by the executable instructions to at least: establish a content presentation session, wherein individual portions of a content item are provided to each of a plurality of user devices concurrently during the content presentation session, and wherein the plurality of user devices are permitted to submit annotations concurrently during the content presentation session; receive, from a first user device of the plurality of user devices, an annotation associated with a portion of the content item; determine a target quantity of annotations per unit of time to be distributed to individual user devices of the plurality of user devices; select a first subset of the plurality of user devices based at least partly on a quantity of the plurality of user devices and the target quantity of annotations per unit of time; transmit the annotation to the first subset of the plurality of user devices; receive moderation information regarding the annotation from at least a portion of the first subset of user devices; determine that the moderation information satisfies a moderation criterion; transmit a group of annotations to a second subset of the plurality of user devices, wherein the group of annotations includes the annotation; and transmit display metadata to the second subset, wherein the display metadata is associated with display characteristics of individual annotations in the group of annotations.
18. A system comprising: a computer-readable memory storing executable instructions; and one or more hardware computing devices in communication with the computer-readable memory, the one or more hardware computing devices programmed by the executable instructions to at least: establish a content presentation session, wherein individual portions of a content item are provided to each of a plurality of user devices concurrently during the content presentation session, and wherein the plurality of user devices are permitted to submit annotations concurrently during the content presentation session; receive, from a first user device of the plurality of user devices, an annotation associated with a portion of the content item; determine a target quantity of annotations per unit of time to be distributed to individual user devices of the plurality of user devices; select a first subset of the plurality of user devices based at least partly on a quantity of the plurality of user devices and the target quantity of annotations per unit of time; transmit the annotation to the first subset of the plurality of user devices; receive moderation information regarding the annotation from at least a portion of the first subset of user devices; determine that the moderation information satisfies a moderation criterion; transmit a group of annotations to a second subset of the plurality of user devices, wherein the group of annotations includes the annotation; and transmit display metadata to the second subset, wherein the display metadata is associated with display characteristics of individual annotations in the group of annotations. 19. The system of claim 18 , wherein the executable instructions to determine that the moderation information satisfies the moderation criterion comprise executable instructions to at least: determine a threshold quantity of moderation tags that are to be applied to the annotation before the annotation is permitted to be provided to additional user devices; determine a quantity of moderation tags applied to the annotation by the first subset of user devices; and determine that the quantity of moderation tags satisfies the threshold quantity.