One thing writers tend to hear a lot these days is, "You have to have a platform." While that's more true of non-fiction writers than fiction, many agents want their clients to have some internet presence: whether it's a blog, a Twitter account, Google +, a Facebook author page, or whatever. Most people agree that self-promotion on social media is important. But let's talk about the line between self-promotion and being obnoxious.
Following a few simple Do's and Don't's can help you get the most of social media. (This post is directed at Twitter, but I think the general ideas can be applied in other areas of life.)
1. Don't Send Auto DM's: One of the number one things I've seen people complain about is getting an automated direct message immediately after following someone on Twitter. "Thanks for following! Buy my book! [Links]. Since most people who use DMs do it for personal communication, it's a bit frustrating to get what looks like a personal message but is really just spam. Also, the vast majority of Twitter users don't look in their DM box at all, because it's always spam. I once found a message that was 107 days old. I don't know anyone who appreciates these messages, other than the people who use them, and many people I know will automatically UN-follow those people.
And this should go without saying, but never, ever, ever pitch your book to an agent or publisher via DM if you're lucky enough to have one of them follow you. It's completely unprofessional.
2. Do Tweet Something Other than Self-Promotion. I don't know what the magic ratio is, and maybe there isn't one, but if ALL your tweets are links to your books, I feel like I've signed up for all commercials, all the time. I don't want to be inundated with people asking me for money when I'm online relaxing. And, again, if I notice that you never tweet anything but book promotion, don't respond to @ messages, don't favorite or retweet things that aren't about your books - I'm going to unfollow you, and I'm not going to buy your books.
3. Don't Suggest Your Own Book when People ask for Recommendations: Chances are, if I know you and your book is a genre I enjoy, I've already checked it out. It's just awkward when I say "I'm in the mood to read X," and someone responds with "Hey! Buy my book!" And Tweetdeck doesn't let me delete @ messages, so I'm stuck looking at it, which annoys me more.
4. Do Be Engaging. Be chatty. Be fun. Tweet about the types of things that people who enjoy your books would be interested in. Support other writers. Congratulate them when good things happen. Share books you've enjoyed by other writers. If you want people to read your book, do a giveaway. But don't bombard your followers with self-promotion - the only thing you're doing is alienating people who might've bought your book if you hadn't.