Altruism is the users' desire to bridge and maintain relationship with others through conducting certain behaviors, such as gift-giving or asking for help. In the context of websites, providing support, gift-giving and charity are popular examples. 
Altruism can be considered as a strategy for attracting new users. For instance, users may receive a gift from someone that pulls them into the website, and then, they are incentivized to send gifts to friends, eventually creating a great acquisition loop.
Companies have invested in communities to facilitate employee collaboration and customer self-service. Gamification leverages altruism in communities by incentivizing participation. By giving users a platform to engage, they can fulfill their desire to bridge and maintain relationships with others. Trusted users can be given badges that highlight their status. It has been proven that users with higher group identifications are often willing to remain in a group permanently and to strive toward the goals, and devote themselves to group affairs.
^ 1. Hsu, S., Chang, J., & Lee, C. (2013). Designing Attractive Gamification Features for Collaborative Storytelling Websites. Cyberpsychology, Behavior & Social Networking, 16(6), 428-435. doi:10.1089/cyber.2012.0492