Two things to offer here.
Is Stack Exchange "Exploitative"?
You've provided one definition, and then referenced "ethical exploitation" in your answer using that definition:
Noun (1) the action or fact of treating someone unfairly in order to benefit from their work.
eg. "the exploitation of migrant workers", misuse, ill-treatment
You've omitted, however, a secondary definition of the term that's much more applicable here:
(2) the action of making use of and benefiting from resources.
eg. "the Bronze Age saw exploitation of gold deposits", utilization, usage
Stack Exchange is unquestionably exploitative in this way– but so are you! Just as Stack uses our contributions to populate their platform, you "exploit" that very same resource every single time you visit and read something useful. This is intended and by-design; Stack's mission is to "build a library of detailed answers to every question about [topic]". They're building resources to be exploited by everyone, for both your and their benefit. Their business model is fundamentally linked with people finding their sites valuable and worth exploiting; without that, Stack wouldn't possess nearly the clout that they do, nor the large pool of clients that they get to market their premium products to.
But Does the First Definition Apply?
Circling back to the first definition:
(1) the action or fact of treating someone unfairly in order to benefit from their work.
Stack only fits this definition if you can describe the relationship between Stack and its curators as "unfair", and based on what you've written in your question, answer, and comments, you seem to hold that Stack being for-profit is unfair because curators aren't compensated monetarily.
I think that you are vastly underselling the value that Stack offers to everyone with an internet connection, whether curator or otherwise. The main value of Stack Exchange is most certainly not to the wallets of Stack's stakeholders, it's to the millions of users that derive value from Stack's knowledge base every single day. If anything, that's unfair, not that Stack's employees and stakeholders are getting paid.
Stack is not unfairly benefitting from its curators; it's giving them a place to contribute, and providing a valuable service for millions of visitors, free of charge. And all that without the (largely unethical) data harvesting that's commonplace across the internet today.