How do the social media affect the success of charitable promotional campaigns? We use individual-level longitudinal data and experimental data from a social-media application that facilitates donations while broadcasting donors' activities to their contacts. We find that broadcasting is positively associated with donations, although some individuals appear to opportunistically broadcast a pledge, and then delete it. Furthermore, broadcasting a pledge is associated with more pledges by a user's contacts. However, results from a field experiment where broadcasting of the initial pledges was randomized suggest that the observational findings were likely due to homophily rather than genuine social contagion effects. The experiment also shows that, although our campaigns generated considerable attention in the forms of clicks and "likes," only a small number of donations (30 out of 6.4 million users reached) were made. Finally, an online survey experiment showed that both the presence of an intermediary and a fee contributed to the low donation rate. Our findings suggest that online platforms for charitable giving may stimulate costless forms of involvement, but have a smaller impact on actual donations, and that network effects might be limited when it comes to contributing real money to charities.