1. A method, performed by a computer system, for identifying message content to send to users based on the users' language characteristics, the method comprising: obtaining user-generated content for a plurality of users, wherein each content item is associated with a user ID that uniquely identifies the user that generated the content item; extracting language characteristics from the user-generated content; assigning one or more language characteristic scores to each of the users; clustering the users into groups using the language characteristic scores; testing the responsiveness of each group to different message content by sending a plurality of test messages with different message content to at least a subset of users within each group and measuring the response rates to the test messages; associating different message content with different language characteristic scores by identifying a message content to which each of the groups is most responsive and associating the message content with the group; obtaining user-generated content for a new user; extracting language characteristics from the new user's user-generated content; assigning one or more language characteristic scores to the new user; and sending a message to the new user with the message content previously associated with users having similar language characteristic score(s).