Methods, systems, and apparatus, including computer programs encoded on computer storage media, for posting messages on a social messaging platform. One of the methods includes providing a message composition interface; receiving a user request to include a promotion with a composed message; in response to the request, processing the content of the composed message and the user's profile on the social messaging platform to determine one or more social signals; computing an engagement score for each user in a pool of users, wherein the engagement score for each user indicates the likelihood that the user will engage with the message; selecting candidate users from the pool of users; selecting a plurality of target users from the candidate users based on a promotion value and the respective engagement scores; and posting the message on the platform including adding the message to a message stream associated with the selected target users.