Online mentoring can be useful for supporting girls in science, technology, engineering, and mathematics (STEM). Yet, little is known about the differential effects of various online mentoring formats. We examine the general and relative effectiveness of three online mentoring formats, one-on-one mentoring, many-to-many group mentoring, and a hybrid form of the two. All three formats were implemented in different years in the Germany-wide online-only mentoring program, CyberMentor, whose platform enables communication and networking between up to 800 girls (in grades 5-13) and 800 women (STEM professionals) each year. We combined longitudinal mentee data for all first-year participants (N = 4017 girls,M-age = 14.15 years) from 9 consecutive mentoring years to evaluate and compare the three mentoring formats. Overall, all formats effected comparable increases in mentees' STEM activities and certainty about career plans. However, mentees' communication behavior and networking behavior on the mentoring platform differed between the three formats. Mentees in the hybrid mentoring format showed the most extensive STEM-related communication and networking on the platform. We also analyzed the explanatory contributions of STEM-related communication and networking on interindividual differences in the developmental trajectories of mentees' STEM activities, elective intentions in STEM, and certainty about career plans, for each format separately.