One one hand, I'm glad that Stackoverflow is profitable. That someone is making money on it. On the other, I'm peeved that someone made two billion on all free content I and others created for no other gain than imaginary internet points. Without our work, SO wouldn't have been worth much. None of us can objectively claim to have been take advantage of... but still...
All the free content you and I provided is our way of giving back to the community that answered our questions.
That content generates value for SO, but they do provide the infrastructure we use for free to get our problems solved and questions answered, so it seems like a fair deal to me.
If you didn't want anyone to profit from your answers, you shouldn't post them. If your answer is reasonably popular, you can bet it will be used in a product that makes money without you ever getting compensated for that.
That's how crowdsourcing works. I think I'm more concerned over the fact that stackoverflow now needs to earn $2 billion in revenue over the next 10 years in order for this purchase to have been worth making, and we're going to probably be the ones to pay for that.
$1.8B is the value of the business that encouraged this content creation, not the value of the information that has flowed through SO. The developer productivity gained by the existence of SO is probably on the order of trillions.
