It's more of an art than a science for me, but I use a combo of Twitter, Tumblr and Tapastic, and I feel like I'm gaining some traction. Just shouting into the void of social media probably won't work all that well, but you can find ways to get the word out anyway.
1.) With Twitter, I follow a bunch of other webcomic creators and artists, because they're awesome and interesting people - and one lovely consequence of it is that they tend to take more notice of art/comics, and thus retweet and spread the word to their followers - and I do the same for their stuff. Together, we reach more people than each of us alone.
2.) Participate in community events. I'm involved with this year's Tapastic holiday crossover event, which means that readers of other comics have wandered over to mine to follow the story - and some of them are sticking around to read my comic. I also try to participate in the #ComicTalk and #webcomicchat events on Twitter every Sunday - through it, a bunch of people interested in comics get to meet and talk and find new comics. On Tumblr, I've participated in Goblin Week, Witch Week, Magical Girl March, I personally run RPG Week once a year, I'm currently doing a series of 24 illustrations based on my christmas calendar, etc., etc. It's all a matter of reaching out and participating - it's fun, it's interesting, and you're more likely to gain attention there than if you stay in your own corner of the internet.
3.) Being active on the Tapastic forum helps. We're all here because we like comics, so getting people to read your stuff here is easier than to convince random social media users who might not even read comics to start with.
4.) Project Wonderful, as mentioned, is a good way to get started with some straight-up advertisement.
5.) Speaking of using the Tapastic forums - I clicked on your forum profile, thinking I'd have a look at your comic - and I can't, because your forum profile doesn't link to it. You need to put the link in your forum-profile manually, or people won't find it.
