Paying a random consumer $20 does not make them a contractor. Do you think they all got 1099s?
You are a "contractor" if you are providing services under a contract. A contract exists whenever there is a definitive agreement to exchange valuable considerations – even in the absence of a written, signed contract.
But the sign-up for these apps might have included an explicit "signing" phase! (It's even possible that FB/Google asked for participants' SSNs, just in case any payments went over $600.)
(And if they’re under any sort of confidentiality agreement or other conditions on their app usage, they fit under the Apple terms’ concepts of “Permitted Users” and “Internal Use” even better.)