Prioritizing requirements is a crucial ingredient of successful Requirements Engineering (RE). The popular prioritization techniques assume that stakeholders are known and can be mandated to contribute to the prioritization process. This prerequisite no longer holds for many of today's systems where significant stakeholders (end-users, in particular) are outside organizational reach: they are neither known nor can they be identified among the members of the involved organizations. Classic techniques for involving these stakeholders such as polls or questionnaires are neither interactive nor collaborative, which is detrimental for prioritization. Social media enable collaborative prioritization, but fall short in motivating stakeholders outside organizational reach to participate voluntarily. In this light, we are developing the Garuso platform, which combines social media with gamification for motivating stakeholders. While first approaches to employing gamification in RE are promising, research is still in its infancy. Especially, little is known about the influence of the gamification algorithms controlling single game elements on the stakeholders' activities. In this paper we report on a field experiment in which we investigated this influence with Garuso. We found statistically significant differences between different algorithms controlling single game elements on the contributions of stakeholders to the prioritization of requirements.