There are a lot of ways to answer this so I am just going to start rambling and hope I say something that makes sense. First, the international liberal following part, many of the "international" people on this site are Europeans, and they tend to be on the liberal side. I personally believe that this is because during the colonial period Europe sent its brave, daring, hard-working people overseas to colonize other areas, leaving behind cowards, invalids, layabouts, and of course the inbred nobility and royalty that fleeced them all. Modern liberalism is descended from this mix of leftovers. Instead of nobility and royalty we have the privileged class who want to "help" the lower classes by controlling their lives, and instead of layabouts who leech on society we have, well I guess that part didn't change much.
Anyway, the bottom line is that many of the people who have conservative traits, (hard-working, desiring freedom, distrust of corrupt authority figures, willing to take risks) left Europe. So that is why I think that Europe is more liberal than many other parts of the world.
Now as for the liberals in the U.S. Well that is just common sense, Younger people are more liberal than older people and younger people are more likely to spend time online. People who spend time outside instead of online are more likely to be conservative. People who love to sit around and bitch are more likely to use a site like this, and people who sit around and bitch are more likely to be liberals. People who have jobs that keep them busy are more likely to be conservative, and therefor have less free time to spend online. I am convinced that conservatives are actually far more common than it seems, but they aren't as outspoken because they are busy keeping the wheels of society turning.