Yes, it has.
However, it's a bit more subtle than that. Yes, Stackoverflow is making some money, but everyone who contributes gets a few things out of it too:
1. They get the use value of the overall result. This is a non-trivial value. Linux operates under similar dynamics. A lot of "free" code was written, and it was written to improve on the use value in the body of code they found at the time.
Anyone finding Stackoverflow sees a lot of use value and they may well see improving on it in a similar way.
2. Reputation. I like to think of this as "feed the net and the net feeds you." Back in the USENET days, I was very highly active on SGI computers. Sometimes I wondered about the overall value of that. Then I got stuck on some painful Oracle installation issue. All I had to do was ask, and within an hour I got a nice script mailed to me along with a thanks for all the filesystem / disk questions I sorted out for people. (thanks Jerry)
Given the dollars in play at the time, that script was worth a ton!
3. Network, similar to reputation. My many interactions on the net has often resulted in knowing people who I could help or get help from or start projects with.
Clearly, this all must be balanced and weighed by everyone involved. People who do too much might seem over exploited or suffer opportunity costs of various kinds. And we all see what is worth what a little differently too.
But, it's not exactly free. There is value exchanged and accumulated. Making sure that happens is worth something. Maybe it's worth about what Stackoverflow is taking in.
USENET worked, until it didn't. And when it didn't, it all went down hard. Huge value lost. Maybe that won't happen this time, and frankly I think that's worth a profit to somebody because the use value is high enough to be painful should it go missing.