1. A method comprising: identifying a plurality of hashtags associated with a set of messages in an online system, the set of messages including a message having a textual string including a first hashtag and a second hashtag of the plurality of hashtags; selecting a set of hashtags for which to train a plurality of hashtag classifiers by using a filter, wherein the filter excludes at least the first hashtag and includes at least the second hashtag; training the plurality of hashtag classifiers for the selected set of hashtags, each hashtag classifier corresponding to a hashtag in the selected set of hashtags and trained with messages in the set of messages that include the corresponding hashtag, each hashtag classifier providing a classifier output indicating whether a message should be associated with the hashtag corresponding to the hashtag classifier; identifying one or more hashtags for a subject message of a user of the online system based on the classifier outputs of the plurality of hashtag classifiers applied to the subject message, wherein the one or more hashtags indicate a topic of the subject message based on text included in the subject message; selecting a candidate story likely to be of interest to the user using the topic indicated by the one or more hashtags; and sending the candidate story from the online system to a client device for presentation to the user.