Danmaku is a new commentary design for online videos. Unlike traditional forums where comments are displayed asynchronously below a video screen in order of when the comments are posted, danmaku comments are overlaid on the screen and displayed along with the video. This new design creates a pseudo-synchronous effect by displaying asynchronous comments with certain video segments in a synchronous fashion, and the links between danmaku comments and the video segments are defined by users. Danmaku is gaining popularity; however, little is known, compared to the traditional forum design, regarding how effective the new danmaku design is in promoting social interactions among online users. In this work, we collected 38,399 danmaku comments and 16,414 forum comments posted in 2017 on 30 popular videos on Bilibili.com. We compared user participation from different perspectives, e.g., number of comments, sentiment of the comments, language patterns, and ways of knowledge sharing. Our results showed that compared to the traditional linear design, the danmaku design significantly promoted user participation, i.e., there were more users and more comments in danmaku. Additionally, active users posted more positive comments, though they were anonymous; more linguistic memes were used in danmaku, suggesting that it was used to facilitate community-building. In addition to its effectiveness in promoting social interactions, our results also show that danmaku and forum designs play complementary roles in knowledge sharing, where danmaku comments involved more explicit (know-what) knowledge sharing, and forum comments exhibited more tacit (know-how) knowledge sharing. Our findings contribute to the development of social presence theory and have design implications for better social interaction via online videos.