Twitter. Twitter. TWITTER.
Check out #WebcomicChat #ComicArtistsUnite #ComicBookHour and more. A lot of people don't really understand how powerful a tool Twitter can be, because it looks kinda superficial on the surface. The best way to use twitter is to think of it as a room filled with people at a party - you can only start talking to people if you jump into conversations. So go start chatting with people!
Also Facebook groups! But be careful. In facebook groups the best way to get your stuff noticed is by being active within that community beyond "Here's my stuff go read it." You've got to talk with the group members, provide reviews, offer advice ect.
Social Networks are all about building up social credit (ie. making friends) and spending that social credit when you absolutely need it.