I've been writing professionally since 2011, and started using social media in 2014, pending the publication of my first book by Compass Books, called 'Freelance Writing on Health, Food and Gardens'. My day job is writing magazine articles. I've also self-published a second writing book, a collection of WWII memoirs, and some inspiring Christian stories.
When I first joined Twitter, it struck me as a waste of time, but I persevered and have since come to enjoy some of the interactions with other authors. I've learnt some valuable things from people on social media. For example, I first heard about the Author's Licensing and Collecting Society (ALCS) from people I met on Twitter, and I've since made money from joining ALCS, which made the whole experience worthwhile! I've also picked up things about commercial book publishers, agents, and self-publishing, which have been useful.
I find Twitter infinitely more engaging than Facebook, perhaps because my Facebook author page rarely appears on anyone's feeds due to Facebook's policy of charging for exposure. Twitter, in contrast, results in some great interactions. I've met some really interesting people, and find the author community really supportive. I try to post links and tips that will help people achieve their writing ambitions.
My blogs help me engage with readers, exchange thoughts, and promote my work. Does my social media activity result in book sales? Well, writers like to read too, and some of those people I've met online have bought copies of my books. Some even told me how much they enjoyed the books or felt inspired by them. This and the other benefits, such as generation of feature ideas and positive interactions, make logging onto social media regularly, worthwhile.
Authors can be a generous bunch on social media, and will offer hints and tips, help promote your work, might give you an interview on their blog, or support you in other ways, so there are good reasons to be active on social media. My most memorable interactions on social media have been from readers who have loved my books, or been inspired by my articles, and have tweeted to say so.
The main thing to remember is that it's not a one way street. If your communications are all about promoting your work, to the exclusion of everything else, you're not making the most of the opportunities to engage with potential readers in a more meaningful way. Two way communication is more effective, more rewarding, and more likely to generate interest in your work.