Some good points about contributing your time for free, but the comparison is a bit inflammatory. Sharecroppers had no real choice, lots of risk, and such an arrangement was their only income.

Most instances of investing your time in an online activity are not so vital to one's financial income.

Contributing to stack overflow helps other programmers, gives you a some exposure, and perhaps answers questions of your own. Founders of SO put up capital and energy to build and maintain the site, and don't claim ownership of user-contributed content (cc-wiki license), and get revenue from advertising.