You're just reading the title and I can already hear your skull popping like a corn kernel as your brain explodes.
It not a terrible thing. In fact, it's something that can be fun if you use a methodology you enjoy. But like the snail mailing lists of yore it can be highly valuable in the long run.
There's always social media, such as Twitter, Facebook, et al. Some writers love them; some don't. And if you don't, you're not going to engage which makes the whole exercise pointless.
Also, which one you use really depends on your market. Adults hang out on Facebook. Teens and college-age young adults tend to hang out on Instagram and Snapchat (aka fewer old people). Since I write for adults, I'm going to focus on those avenues.
Overall though, don't just hawk your books. Actually talk to your readers. About the weather. About books in general. About your favorite TV shows and movies. (Seriously, Alter Ego got a lot more hits on her turn on a recent multi-author giveaway on Facebook because she included a certain comic book character t-shirt in her grand prize bag.)
A prime example of the "BUY MY BOOK!!!" phenomenon is Twitter. I unfollowed a bunch of people because that's all I saw. The other issue I see is writers following other writers, then getting pissed when the followed writers don't follow in return. This was bad advice in 2011; it's worse advice now. Add in Twitter's tweaking of its algorithm and charging for top placement, and you have an unviable situation made even worse. It's gotten to the point my interaction on Twitter is practical non-existent.
A lot of writers swear by Pinterest. I'm not doubting their results, but I have extreme reservations about using it thanks to its original, draconian provisions that stole picture copyrights. Supposedly, this have been rectified, but that kind of heavy-handed tactic makes me leery about using them.
While DH made a fan page for "Suzan Harden" on Facebook, I rarely use it. I have more personal friends checking it out than readers.
The only social media I truly and regularly use is Alter Ego's Facebook account to interact with readers and other romance/erotica authors. Alter Ego has participated in a couple of giveaways. While neither resulted in a ton of sales, it allowed me to introduce myself to new readers, and kept Alter Ego visible during the two years of no new releases.
Then there's the good old fashioned mailing list. There's numerous ways to do this--MailChimp is one of the most popular. Alter Ego sends out a quarterly newsletter (or tries to) and attaches a free short story. There's a ton of other ways to handle mailing lists. Look at what other writers do, and choose the method that works for you.
While the ultimate goal is to sell more books, you can do so by treating your readers like real people. In doing so, you're already miles ahead of the traditional publishers.