Here's Gentzkow and Shapiro's breakdown of who visits the ten top conservative and liberal sites:
Conservative sites visitors
Liberal sites visitors
Assuming we trust their data, one explanation for the imbalance could simply be that there are tons more conservatives visiting all political sites; if that were the case, even if liberals and conservatives had similar biases toward consuming like-minded media, we'd still see more conservatives on liberal sites. But my guess is there aren't that many more conservatives lurking online.
Another possible explanation is that - as suggested in the title - conservatives are more open-minded than liberals. Either that or they are more willing to check out what the opposition is saying. That story is somewhat plausible; other research suggests that savvy ideologues are more inclined (than less informed ideologues) to click on "counter-attitudinal" articles because they're more comfortable debunking any opposing views they may happen to bump into.
Still, it's curious why you'd see so many more conservative ideologues venturing into liberal waters than vice versa. The final explanation I can come up with - and one that suits my lefty leanings - is that "liberal sites" aren't as liberal as "conservative sites" are conservative; that is, that a conservative visiting DailyKos is more likely to come across information he can jive with than a liberal is going to find on RushLimbaugh. In other words, liberal websites - as opposed to conservatives readers - are the ones who are open-minded.
Any other interpretations out there?