For Twitter, you just need to hit on the right hashtag trends. Ask yourself, what is your comic about, then go to Twitter, start doing a search on that topic, and see what's trending in that category. Then Tweet your comic tapas link and how it fits in with the topic. For example, our comic is "Insignificant Otters":
And we noticed "otters" happened to be trending today, at about 1000 tweets in the past hour. Perfect time to Tweet about the comic, toss in the trend word "otters", and we got two new subscribers just now.