  As large-scale international collaborations become the standard for astronomy
research, a wealth of opportunities have emerged to create innovative education
and public outreach (EPO) programming. In the past two decades, large
collaborations have focused EPO strategies around published data products.
Newer collaborations have begun to explore other avenues of public engagement
before and after data are made available. We present a case study of the online
EPO program of The Dark Energy Survey, currently one of the largest
international astronomy collaborations actively taking data. DES EPO is unique
at this scale in astronomy, as far as we are aware, as it evolved organically
from scientists' passion for EPO and is entirely organized and implemented by
the volunteer efforts of collaboration scientists. We summarize the strategy
and implementation of eight EPO initiatives. For content distributed via social
media, we present reach and user statistics over the 2016 calendar year. DES
EPO online products reached ~2,500 users per post, and 94% of these users
indicate a predisposition to science-related interests. We find no obvious
correlation between post type and post reach, with the most popular posts
featuring the intersections of science and art and/or popular culture. We
conclude that one key issue of the online DES EPO program was designing
material which would inspire new interest in science. The greatest difficulty
of the online DES EPO program was sustaining scientist participation and
collaboration support; the most successful programs are those which capitalized
on the hobbies of participating scientists. We present statistics and
recommendations, along with observations from individual experience, as a
potentially instructive resource for scientists or EPO professionals interested
in organizing EPO programs and partnerships for large science collaborations or
organizations.

Physics Education;Instrumentation and Methods for Astrophysics