1. A method comprising: identifying a plurality of hashtags associated with a set of messages in a social networking system; applying one or more exclusionary filters to identify a set of eligible hashtags by excluding hashtags in the plurality of hashtags that meet at least one of the exclusionary filters; training a plurality of hashtag classifiers, each hashtag classifier corresponding to an eligible hashtag in the set of eligible hashtags, the hashtag classifier providing a classifier output indicating whether a message should be associated with the hashtag corresponding to the hashtag classifier; and identifying one or more hashtags for a subject message based on the classifier outputs of the plurality of hashtag classifiers applied to the subject message.