Estimation of true audience size for digital content

A content server system provides a client device with content, such as an audio stream. Using various techniques, an estimate is made of the actual size of an audience associated with the provided content, rather than assuming that the audience is limited to a single user of the client device. The estimate may be made by the content server system, the client device, or the content server system and the client device collectively. Using the estimate of actual size of the audience, the content server system can take actions appropriate for the audience, such as providing advertisements appropriate for the size of the audience and for the collective characteristics of the audience members. The estimate of the actual audience size additionally allows the content server system to be compensated more precisely for any advertisements provided to that audience.