I would personally guess that the internet itself leans left. You’re going to find more computers in urban homes than in rural, and urban areas tend to be more liberal. The internet is also home to younger people, who are generally more liberal than older.
The Straight Dope, as someone posted, is printed in an alternative newspaper that’s probably most popular in liberal cities.
Smarter people favor skepticism, making them generally less strongly religious or otherwise fanatical. Without strong religious views, it’s hard to maintain strong socially conservative values.
Conversation and friendly interaction, similarly, makes it hard to hold bigoted views in regards to homophobia. So you’re not going to find a lot of homophobics on a site with a thriving gay population, and most homophobics are, in modern time, conservatives.
Smarter republicans are more likely to consider themselves to be too busy to waste time chatting on the internet and debating with loonies.