A lot of writers want to know if they NEED to be on social media. But honestly, that's the wrong question, because "need" can mean a lot of things. You'll likely remain a physically sustainable individual (i.e., you won't fall over and die) if you aren't on social media, but that's not really what writers are usually after when they ask that question. (Note this issue is handled differently for non-fiction authors. I'm addressing fiction here.)
What they're actually trying to figure out probably looks more like this:
Is a social media presence necessary for me to sign with an agent or to receive a publishing deal?
No. I suppose a rare agent or editor requires a certain level of social media activity, but for the majority, the book is the deciding factor in fiction, not your internet prowess. Many agents and editors do really like to see authors who are willing to engage and who have a basic level of competency on a social platform or two.
Will it be bad for my career if I don't have a web presence?
No web presence at all? Yes. Readers search online for ways to stay updated on and connect with authors they like. Not being able to find that online may mean a portion of your readership loses sight of you, and a good portion of your potential readership won't ever find you. Authors need, at the minimum, a solid, informative website.
Will it be good for my career if I have a web presence?
Assuming you aren't making the mistakes that can actually turn readers away, yes, it will be good for your career. Even if you don't blog, a website allows readers to find you and check in with your current and new upcoming books, find out about any appearances you're making, and read similar news. Also, having a friendly and professional social media presence will generally help you make career contacts and prompt readers to check out your book, and it's a great way for authors you come to know and people you meet at conferences and signings to keep up with you. You can also learn a lot about writing and career choices from being part of the community, and that is definitely good for your career.
Can a social media presence benefit my sales numbers?
Posting requests for people to buy your book probably won't result in any kind of notable increase in sales numbers, so people often answer "no" to this question. But being a friendly and helpful member of a social media community can definitely build your platform. So, yes, but usually indirectly. Word-of-mouth is the best tool for book sales, and that's what social media is all about. I buy books from authors I respect and enjoy interacting with on Twitter, and I buy books those authors recommend. The effect of your social media presence on your sales should be a slow, steady build that's focused not on receiving money but on developing relationships with readers and industry professionals. Those contacts and relationships developed there also frequently give authors opportunities they wouldn't have otherwise.
There's too much to know and too little time to do it in; what do I really need to know?
Look around at Goodreads, Pinterest, tumblr, Instagram, Twitter, Facebook, etc. Pick a communication style that you like and is natural for you (they all have a different approach-- 140 characters, short posts, pictures, etc) and sit back and watch what happens there. Don't rush yourself; it's fine to go slow. There's no race to developing a social media presence. Get comfortable, and then try it yourself.
My main social media advice to my clients is this:
Be genuine, be intentional, and be limited.
Be genuine: it's okay to be yourself. Show some personality, give us some insights on who you are and what you love. It can be tough to see personality online, so make a point of being yourself.
Be intentional: think about what you are posting before you post, make sure you know what you're doing with the platform, and guide your content by making it recognizable. I like to tweet #subtips and book recommendations. Agent Jessica Sinsheimer posts a lot of recipes and food pictures in addition to her agent advice. NYT bestselling author Jennifer Weiner live-tweets The Bachelor. This works best when it's done naturally, a result of things you already love, rather than as a ploy to get people to buy your book.
Be limited: Some of us have extreme areas to our personalities. Some of us have a hard time with negativity, or ranting, or speaking before we think (hand in the air for me on that last one). Some of us just really love taking photos of our lunch. Limit how much of that you put out there. The internet is forever. Before you engage online, imagine you're saying it before the whole world, because you are. Don't let that scare you, but do let it inform your posting habits.
The other half of being limited is limiting the time you spend on social media. Writing a great book is always, always the best thing you can do for your career as an author. Everything else is supplementary. Supplementary things help, and many of us need that help. But they aren't the main thing.
So what's the straight talk on social media? It is a smart, proactive choice for authors to learn to comfortably and effectively use a social media platform or two. Plus, it can be fun if you take a little time to find what works for you and be comfortable with it, and you can learn and share a lot in the community there. In general, writers are an awesome group of people and we love talking to other book people. Social media is a great way to make that happen.