There are many different instances where the social contract of using the stake for Steem was defined.

For example, in the 2017 Roadmap (https://steem.com/2017roadmap.pdf)

Decentralizing Stake
The Steemit, Inc.-controlled primary account, @steemit, which holds approximately 41% of the platform’s Steem Power, will be gradually divested of its holdings in an effort to increase promotion and development of the platform, and this distribution shall further the platform’s security through decentralization of voting power.

Several methods will be employed, including and not limited to: funding the continued research and development of steemit.com, the Steem Blockchain’s first and best application, promoting and publicizing the Steem Blockchain and features, hosting highly available services for platform users, sponsoring conferences and community gatherings, and sponsoring undertakings to build applications and increase user adoption across the entire ecosystem.

This will likely be a multi-year process, but is included in this 2017 Roadmap for the sake of clarity and understanding surrounding our organization’s plans and goals. We have several very large initiatives currently in research serving this goal, and the community should expect further announcements later in the year regarding our firm plans as it yields fruit.

Ultimately, the goal is to harmoniously align the interests of all participants across the entire spectrum of people using and trading in STEEM.

Here is a video where the social contract is mentioned: