In one embodiment, a method includes identifying an emergency event; determining that a threshold percentage of users who are associated with the emergency event have posted content related to the emergency event to an online social network; sending, in response to the determination, a safety-check prompt to each of a first set of users; determining a current-prompting probability based on one or more engagement metrics of the safety-check prompt by the first set of users; and sending, in response to determining that the current-prompting probability is above a first threshold probability, the safety-check prompt to each of a second set of users, wherein the second set of users comprises more users than the first set of users.