We introduce a framework for personality-targeted design. Much like a medical treatment applied to a person based on his specific genetic profile, we make the case for theorydriven personalized UI design, and argue that it can be more effective than design applied equally to the entire population. In particular, we show that users' conscientiousness levels determine their reactions to UI indicators of critical mass. We created a simulated social recommender system in which participants answer a short personality questionnaire and are subsequently presented with a picture of a pet that purports to be the "best match" for their personality. We then manipulated the UI by providing indicators of the existence and the lack of critical mass. We tested whether the interaction between personality and UI design affects users' participation. The findings validate our hypothesis, showing that manipulation of the critical mass indicators affect high-conscientiousness and low-conscientiousness participants in opposite directions.