Well, for one thing, its because the itnernet is one of the few safe places atheist can actually voice their opinion, without being ridiculed, or, indeed, seriously hurt. Especially in countries where religion is dominant, such as the U.S., Spain, Itally, etc. So, it's no surprise, really. And there's the fact that, it's not so much the Internet being populated by atheists, rather tnan atheists being the ones who grew-up with it, and therefore feel more at home. I dunno, just my thoughts.