There is a time and a place for paid advertisements. You are going to draw a lot more eyeballs onto your work with some good paid advertising that you ever will with free advertising.
That's not to say that free advertising isn't worth it though. It's absolutely worth the time, energy and growth you can see over time with your comic. It just won't happen all at once. You'll get a little trickle.
There are a few ways to make advertising work FOR you in the best ways possible. There are some things you should be doing for free even if you pay for advertising. These are the best I've found.
1. Engage with your Facebook friends and twitter followers (and Instagram, and Pinterest, and Google+ too). The #1 way to build a loyal following is engagement, and the best way to deal with engagement with people. It's the #1 stat Facebook uses when they determine who sees your posts. And the best way to build engagement is to talk to your friends on facebook and twitter. Like their photos, retweet their stuff, actually, you know, give a shit and show it. Then, when you post something, they will like it too and you'll build a little community online just for you. Important: Please don't pretend to give a shit. Actually give a shit.
2. Tweet at people on Twitter that you don't know. When you see something cool online, tweet at the person that's sharing it and try to engage in a discussion. Again, it's important to actually give a shit or two about what's going on. Nobody likes a liar.
3. Find a famous person in your niche and like their followers. It's a way to introduce yourself to those followers, who already like somebody else doing what you do. Maybe they will follow you back, maybe they won't. But hopefully they remember you. Only do this with 20-100 people a day though.
4. Post free ads on Project Wonderful. It's actually wonderful. I hate giving up this secret because if more people know about it, less promotion sites will be free. But I love giving it up, because it will help you gain more web traffic, and I want you to succeed! Basically, you can create and bid on all sorts of advertising on project wonderful, including sites that have free advertising. It's a bidding system so you'll be outbid a lot, and you can only keep free bids up for 2 days, but it's a great way to build your brand for free.
Man. I feel like I barely scraped the surface of free promotion. I could go on and on forever. Maybe later.
Make sure to check out freekickstartercourse.com too in order to start your journey on the right foot.