Twitter is widely used by software developers. But how effective are tweets at promoting open source projects? How could one use Twitter to increase a project’s popularity or attract new contributors? In this paper we report on a mixed-method empirical study of 44,544 tweets containing links to 2,370 open-source GitHub repositories, looking for evidence of causal effects of these tweets on the projects attracting new GitHub stars and contributors, as well as characterizing the high-impact tweets, the people likely being attracted by them, and how they differ from contributors attracted otherwise. Among others, we find that tweets have a statistically significant and practically sizable effect on obtaining new stars but only a small average effect on attracting new contributors. The popularity, content of the tweet, as well as the identity of tweet authors all affect the scale of attraction effect. In addition, our qualitative analysis suggests forming an active Twitter community for an OSS project or OSS developers maintaining strong Twitter ties plays an important key to attract new committers by tweet. We also report that developers who are new toGitHub, have a long history of Twitter usage but few tweets posted are more likely to be attracted as contributor to repositories mentioned by tweets. Our work contributes to the literature on open source sustainability.