This study explores how observational learning and social-network Word-of-Mouth (WOM) affect sales. Using a panel data set consisting of more than 500 deals from Groupon.com, we find that not only can both mechanisms help increase sales, they interact positively to generate additional sales. This complementarity result suggests observational learning and social-network WOM provide different types of informational signals to influence sales. While observational learning primarily updates consumer beliefs about a product, the predominant effect of social-network WOM is through advertising that complements the signaling effect of observational learning. We also compare these effects between search goods and experience goods. We find that observational learning and social-network WOM continue to be complements in both cases. However, compared to social-network WOM, observational learning has a stronger effect for experience goods than for search goods.