I went through a phase where I found and referred loads of photographers thinking the extra 3 cents from their sales would make a pleasant little top-up sum.
My findings ...
About 4/5 of those I referred either didn't get accepted, or if they did, they never opened a portfolio.
And of those few who did get accepted and uploaded , almost all of them uploaded no more than 10-12 photos and then seemed to give up. Not one is a regular contributor.
So ... I get a few cents every day from their work, but hardly worth the effort.
On the other hand, if this is typical, it must be very good business for SS ... loads of small portfolios of images that sell every now and again, but taking forever to reach each individual's $100 payout point.