While public engagement with science activities traditionally inhabits physical environments (i.e., museum exhibits), as the Internet becomes more ubiquitous, new types of public engagement with science mediated through information technologies have emerged. Instead of having scientific findings filtered through traditional mediators, scientists have begun to take advantage of social media in order to communicate directly with the general public. This paper focuses on technology mediated public engagement with science in an online environment, specifically the sub-Reddit called "r/science", on a popular platform, Reddit, in which we investigated the factors contributing to user engagement and perceived effects of science communication from the users' perspectives. The survey instrument including user engagement scales, perceived effects of science communication, and demographics were distributed among 2000 participants in the r/science Ask Me Anything (AMA) series. We analyzed 146 survey responses using descriptive statistics and ordinal logistic regression. The findings indicated that the participants were generally engaged compared to ones in other studies that used the same user engagement scales and perceived positive effects on science communication, except when it came to building trust. Furthermore, we found that time spent on this particular platform appeared to be the most important factor when it came to positive perceived effects of r/science AMAs. This type of mediated public engagement has been insufficiently investigated, most particularly in terms of the examination of participants' perspectives. This void is addressed in this study. The findings from the study will also be informative to similar platforms that support mediated public engagement with science.