1. A computer-implemented method comprising, as performed by a content management system comprising a server computing device configured to execute specific instructions: establishing 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; receiving, from a first user device of the plurality of user devices, an annotation associated with the content item; determining a target amount of annotation content per unit of time to be distributed to individual user devices of the plurality of user devices; selecting 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 amount of annotation content per unit of time; transmitting the annotation to the first subset of the plurality of user devices; receiving moderation information regarding the annotation from at least a portion of the first subset of the plurality of user devices; determining that the moderation information satisfies a moderation criterion; and transmitting a plurality of annotations associated with the content item to a second subset of the plurality of user devices, wherein the plurality of annotations includes the annotation.