If the typical person has 5 friends actively playing, and there are 5 different parts, the chances that a person can get one piece from each friend is 3%? There

If the typical person has 5 friends actively playing, and there are 5 different parts, the chances that a person can get one piece from each friend is 3%? There were also limits on the amount of times that players can help friends daily; it encourages a tit-for-tat dynamic of helping only those who help you. But without built-in tools to track this (mark fav friends, etc), there was always some tension about who "deserves" your help, and players would not spend all their help options right away so they could save some to give back to helpers later in the day, etc... All of these simple mechanics (or lack thereof) interact to create additional scarcity in a very natural, subtle and organic way. And all of it guided, adjusted and optimized via metrics and A/B tests. (different games worked differently and evolved over time, the above is not meant to be a precise and exhaustive description)