Today businesses and private households worldwide rely on Free Libre Open Source Software (FLOSS). Most FLOSS-projects however are threatened in their existence due to a lack of sustained contributors. The early identification of developers who are likely to remain at the project is an eminent task for the management of FLOSS-initiatives. Previous research showed that the subjective judgment of individuals is often inaccurate emphasizing the need to objectively assess retention behavior. Consistent with the concepts Person-Job (P-J) and Person-Team (P-T) fit from recruitment literature, we derive objective measures to predict developer retention in FLOSS-projects. To evaluate our proposed measures we assess the retention of former Google Summer of Code (GSoC) students. The results show that students' prior level of project experience, expertise and communication intensity correlates strongly with their ongoing participation. Surprisingly, our analysis reveals that students with abilities that are underrepresented in the project do not remain considerably longer.