In one embodiment, a method includes receiving a content posting from a device associated with a first user, the content posting including data representative of a content instance and a geographic location associated with the content instance; determining that a second user is associated with the geographic location associated with the content instance, wherein the determination is based on a first client device associated with the second user being or having been within a predefined proximity of the geographic location associated with the content instance; and authorizing, based on the second user being associated with the geographic location associated with the content instance, a second client device associated with the second user to submit an annotation to the content instance.