Theories on the emergence and maintenance of cooperation expect private expressions of gratitude to confirm bonds of reciprocity between people, bonds that uphold cooperation. Yet in large networks of cooperation and exchange, people also contribute to common goods through upstream reciprocity that spreads to people they may not know. Scientific theories expect that expressions of gratitude are indirectly caused in these settings when people receive benefits from others and communicate back their feelings and their sense of obligation. Might those expressions of gratitude also move downstream through networks, widening the spread of thanks and extending contributions to a community? In this talk, hear preliminary results from a participatory field experiment with Wikipedia communities in the Arabic, German, Polish, and Persian language editions. Working alongside hundreds of volunteers across multiple languages, we randomly assigned over fifteen thousand Wikipedia contributors to receive private messages of thanks from other contributors. We then observed their continued contributions and rate of thanks sent to others on Wikipedia. Overall, this collaborative study with Wikipedia contributors investigates the role that expressions of gratitude play in the diffusion of appreciation and the maintenance of public goods.