We designed an online field experiment to examine the impact of social–media generic advertising related to walnuts on consumers’ purchasing behavior and willingness to pay (WTP). Volunteers (n = 751) were assigned into three different groups with one control and two treatment groups, shown advertisements in the form of Twitter posts, and surveyed. The control group was shown ten coffee advertisements rather than walnut advertisements. The two treatment groups randomly received either ten posts related to the health benefits of California walnuts or recipes using walnuts. After browsing the posts, participants were asked to reveal their WTP for ten different products, including raw walnuts, salted walnuts, unsalted walnuts, raw pecans, raw cashews, and more. Our results suggested that participants in the two treatment groups had higher WTP for walnuts than participants in the control group. Moreover, posts about the health benefits of walnuts increased subjects’ WTP more than posts with recipes using walnuts. In addition, the walnut–related posts also increased participants’ WTP for pecan and cashew products. This indicates that social–media generic advertising has a positive spillover effect on pecan and cashews.