1. Facebook - I've created a group page for Champion City Comics which can be found here, but I've joined some group pages dedicated to webcomics and they have been helpful in promoting my webcomics. Overall, Facebook has been a great site to use for promoting webcomics and developing professional contacts.
2. Twitter - I recommend all webcomics creators use Twitter to promote their webcomics. Twitter and Facebook bring in plenty of traffic for my webcomics. You can follow me at @TonyDougWright.
3. Reddit - On a bad day Reddit brings me 1 pageview, but once in a blue moon I can get 2,000 pageviews in a day. Reddit is a tough crowd to win over, so good luck.
4. Linkedin - It's a place for professionals to develop contacts, but it has generated some traffic for my webcomics.
5. Google+ - This site seems to be the Betamax of social media. I still post my webcomics updates at Google+.
6. The Webcomic List - Join the forum and introduce your webcomic to the members. They can be a tough crowd, but they do have some very helpful members.
7. Comic Vine - They have a general forum, but nothing really dedicated to webcomics. If your webcomic is a pop-culture webcomic then you could take advantage of forums dedicated to Marvel or DC characters.
8. Comic Book Resources - I've used their forums to promote webcomics with some success. The INDEPENDENT forum is where I post my webcomics announcements.
9. Comic Related - This was the first site to promote Champion City Comics and I will use their forums to promote webcomics.
10. Digital Webbing - You can promote your webcomics in the forums, but this is a good place to find an artist or a writer.
Tony Wright is the owner and editor of Champion City Comics. Check out his webcomics Day 165 and The Red Devil at Tapastic.