Online discussion websites, such as Reddit's r/science forum, have the potential to foster science communication between researchers and the general public. However, little is known about who participates, what is discussed, and whether such websites are successful in achieving meaningful science discussions. To find out, we conducted a mixed-methods study analyzing 11,859 r/science posts and conducting interviews with 18 community members. Our results show that r/science facilitates rich information exchange and that the comments section provides a unique science communication document that guides engagement with scientific research. However, this community-sourced science communication comes largely from a knowledgeable public. We conclude with design suggestions for a number of critical problems that we uncovered: addressing the problem of topic newsworthiness and balancing broader participation and rigor.