I was wrong about the WSJ's circulation on Tuesday; I got confused by some slightly misleading language in the New York Times. If and when WSJ.com goes free, the newspaper will still have a huge circulation lead over all other US newspapers, bar USA Today. Here's a chart I made, which should explain how things really are:
The blue bar is total newspaper circulation, which is just over 2 million. Of that, around 350,000 newspaper subscribers (the light blue part) also subscribe to the website. (Interestingly, the vast bulk of newspaper subscribers do not have access to the website.) The red bar is subscribers to the website who do not take the newspaper.
If and when WSJ.com goes free, then, the red part of this chart will disappear. But the people who presently subscribe to both the newspaper and the website will still (one assumes) continue to subscribe to the newspaper, which means that total newspaper circulation should stay more or less at the 2 million level.
To be sure, there will be some newspaper subscribers who decide that if all the content of the newspaper is available online for free, there's no point in paying for the paper. So circulation is likely to fall a little. But not enormously.