I work for a digital marketing agency - but as far as I know we haven't had any indie game teams as clients, and what works for one industry won't necessarily work for another. I know this doesn't answer your question, but maybe this can provide some context for other answers:
Ultimately it comes down to identifying your goal(s) (usually sales, downloads, etc.) and driving traffic to your site/fb/app store page to meet those goals. That makes it sound a little sterile but ultimately there are people out there looking for games, so it's worth putting some thought into where those people are looking, what they're looking for, etc. so they can find you.
Some basics: Analytics: Plug google tag manager into your site, then add google analytics via tag manager. Do this first so you can see improvement.
Do some keyword research: Google's keyword planner is the de facto free tool to use for this. Make a list of terms that are relevant to you. Aim for high search volume, low competition. You'll refer back to this research in pretty much everything else you do.
Optimize your site for SEO: make sure the H1s, titles, content, etc. are relevant for the terms/phrases you researched.
Publish content: Write blogs, generate art, trailers, spotlights, etc. Create a calendar of content to publish. Try to squeeze as much out of each piece of content as possible (post it to various mediums, repost it after a month or two if relevant, experiment to see what is effective). For blog topics: could be on the tools your team uses, reflections on the dev process, a review of concept art -> finished product, etc.
Network: Get backlinks (links from other sites back to your site). It's probably the most important thing you can do, but also tends to be the most difficult. Offer to write guest blogs, do interviews, QAs, press releases, etc.
PPC (pay-per-click) ads: PPC usually means google ads - you'll write ads and bid for keywords, people will see them when they search for those keywords, then you pay when they click the ad. Great if you can get it to a point where $1 in equals $2 out, but can take a little (or a lot) of work to tweak the audience, keywords, etc. to get it to convert right.
If you (or other indie game folks for that matter) are interested, I'd be down to swap marketing work if you'd let me write up a case study about the project. It's worth it for us to get the experience in a relatively unknown vertical, and in the end it's a piece of content we can potentially publish elsewhere for mutual benefit. Just a thought.