In some examples, a method includes receiving, from one or more client devices, a stream of messages composed by one or more users of the one or more client devices, wherein each of the messages includes a particular hashtag, determining, using a set of metrics that are based at least in part on the messages, a trending score that represents a magnitude of a trend for the particular hashtag, in response to determining that the trending score satisfies a threshold, sending, to a content provider system, a set of demographic data that describes one or more of the users who associated with the particular hashtag, and, in response to receiving, from the content provider system, targeted content that is based at least in part on the particular hashtag and the set of demographic data, sending, for display at the one or more of the one or more client devices, the targeted content.