Last year Rupert Murdoch reversed himself and decided not to make the online version of the Wall Street Journal free. That looks like a good call: WSJ.com has been posting huge growth this year, even as it has kept its pay wall up. But as we noted earlier this spring, that growth might be a mixed blessing: Because the WSJ is getting many of its new readers “through the side door” – referred by other sites – they are less likely than the Journal’s core audience to stick around and read more stories. That would explain today’s news, which is that while unique visitors have continued to increase for the Journal this spring, those visitors are reading fewer stories than they have in the past. The WSJ told Mediaweek WSJ.com reached 16.2 million unique visitors in June, a 94% increase from last year. Page views were up 45% to 150 million. Impressive, but not quite as impressive as earlier this year, when the Journal trotted out March numbers for Portfolio. In March, WSJ.com saw 15 million unique visitors, a 175% y/y increase, and page views were 165 million, a 75% increase from 2007. That means not only is growth slowing, but monthly page views took a 9% drop from March. We sort of saw this coming: Much more likely is that WSJ.com is getting its links in front of lots of new readers, something that Murray alludes to here: “We’ve gotten much smarter about search engine optimization, and much smarter about working with portals and aggregators.” That also explains why uniques are growing much faster than page views – if someone gets guided to WSJ.com by, say, Digg or Google News (GOOG), they’re much less likely to read additional stories than a WSJ subscriber would. The skeptical way to look at the growth, then, is to point out that WSJ.com’s select, high value audience is getting diluted as it grows, which will eventually bring down CPMs for the site. Again, this isn’t a terrible thing: Most publishers, online or off, would be happy to post triple-digit year-over-year increases in their readership. And even if that shaves the Journals’ ad rates down a bit, Rupe should still come out ahead in the end. But do keep an eye on that Digg-bait. See Also: WSJ.com: Paywall Isn’t Hurting UsWhat Happens To The New WSJ When The Election Is Old News?