Is it really clear? Are the people they were paying technically Google and Facebook employees / contractors?
This is generally false. Minors generally can make valid contracts, though such contracts are usually voidable by the minor prior to execution. [0]
[0] without otherwise endorsing the site as an authority, the discussion here provides a good general coverage of the issue: https://contracts.uslegal.com/contract-by-a-minor/
When you are paid to take a survey, do you magically become an employee of the company conducting the survey?