"Get on social media" is one of the most common bits of advice given to authors looking for ways to grow their platform and promote their books. It is not bad advice, either, as long as you keep in mind two things: one, social media is not a fast track to book sales (for direct sales, AMS ads are far more effective); and two, in all things social, you only get as much as you give.
So here, in my opinion, are the most common mistakes authors make when they try to approach social media as part of their professional platform:
1. Making it all about you - this one is perhaps the most obvious, but it's incredible how many people try to force sales through social media, and/or tire their followers with constant cover reveals, reviews, banners, teasers and promos. I can't think of a more effective way to make people mute or unfollow you. Also very annoying is when you are thrust into some obscure group as soon as you accept someone's friend request. Don't do that!
2. Getting too personal - a personal touch is perfectly acceptable. In fact, it's great. Social media is, after all, about making connections, so there's a place for posting pictures from your latest trip or your kitchen makeover. But if all you ever post are photos of your cat at different angles, it won't contribute much to growing your professional network or enhancing your fan base.
3. Not posting any actual content - memes, games and funny videos will get you some likes and make some people engage with you, but it won't do much for you in the long term. You have to be selective. Seek out content that will educate, uplift, surprise or entertain your audience on a deeper level, and add something personal: a phrase, an anecdote, a comment. It's much better than just sharing or re-tweeting without saying anything. Show that you've actually put a moment of thought into what you're sharing.
4. Being sporadic - many authors are very active on social media in the period surrounding their book launch, and disappear for weeks or months afterwards. It doesn't work this way. The social media world is very dynamic and fast-paced, and you must keep up a steady presence to build and maintain connections.
5. Getting off-topic - I have seen many author accounts where 90% of the time the author posts about how much they hate Trump. Or about dog shelters. Or pollution. This is called getting off track: you open an account for one purpose and get caught in something completely different. Stay focused! I personally try to keep away from politics and hot topics such as gay rights.
6. Trying to do too much - if you are new to social media and open a Facebook account and a Twitter account and get on Instagram and Pinterest and YouTube and try to keep up a podcast and two blogs, you will most likely be overwhelmed. It's better to stick to one or two platforms and really engage there, than do a perfunctory social media rush without the time to really develop meaningful connections.
7. Choosing the wrong platform - what platforms to choose depends both on your personality and the audience you want to address. If you write for teens and want to hang out with your crowd, you most likely won't find it on Facebook. If you love photography, you might find a good place on Instagram. Use your common sense and play to your strengths.
One last thing: I don't spend much time on social media, and there's no real need to. It's more about distribution of your allotted time. Here's approximately how I split mine:
1. 30%-40% - checking out other people's stuff, liking and commenting, participating in group discussions.
2. 20% - sharing non-commercial content - writing posts, uploading photos and videos, passing on other people's interesting stuff.
3. 10%... OK, 20% - goofing off.
4. 10% - Optimizing and updating my profiles, banners and pages.
5. 10% - sharing stuff about my books, including cover reveals, quotes, reviews, giveaways and releases.
I get good response (engagement, sharing, etc) on my commercial stuff because I've "paid" for the right to post it by making it only a tiny fraction of my overall social media activity, and also by helping others promote themselves.