You are undoubtedly correct. The only problem for me is that this Forum in the only social media site that I post to. However, I suppose it is time for me to bite the bullet and create a few social media accounts.
The key to social media, is that you HAVE to use them, otherwise no one will listen when you finally post something (i.e. you build your audience on the site, just like you do here with readers). In other words, don't bother unless you're willing to invest the time building credibility and fans.
At the very least, authors should have a Facebook page. I had one, and used it, until Facebook figured out (due to Google linking my different IDs) that I had duplicate Facebook accounts, one for personal reasons, and one to promote my books. They (FB) combined them, eliminating the tools I used to create the book entries (and have been unable to find and replicate ever since), so now I've largely abandoned Facebook.
The other social media sites I find useful are Google+ (mainly for listing new books, so I tap more than my usual fans), Goodreads, because that's where all the regular readers reside, and it also encourages my regular fans to review my books when I post them to the site, and I've long been a fan of LinkedIn, strictly for the various author forums (since I'm not looking for a job). Since most people there know their future bosses will review their content, they don't engage in the same trollish behavior you find elsewhere (i.e. they're more polite), you've got a bigger group of authors/editors than you do here, and when you post helpful advice, multiple people will check out your website, possibly reading your stories (though you also get the "Do you need a professional editor/web design/book cover" posts.
I haven't tried Twitter, simply because I don't respond to everything under the sun, which seems to be the ONLY way to keep Twitter users engaged.