DO NOT post constantly and only about your book, and where it's on sale, and who read it, and who reviewed it, and on and on. People will tune you out and then it becomes pointless. Authors who have books releasing during a week are given a bit of leeway, but usually because they post about normal stuff 95% of the time.
DO interact. If you like a writer's book, tweet them about it. They'll love to hear it. They'll probably answer. Kiersten White and Shannon Hale--best selling YA authors, peeps!--have tweeted me.
DO make writing friends. It's a fantastic writing community. It's how I got to know Sarah Eden. Even though Krista Jensen and I once lived only four hours apart in Wyoming, I became friends with her through Twitter. She's awesome. I even got to beta read for her a few months ago. It was my friendship with Melanie Jacobson that got me a pitch appointment with an editor at Covenant, who ended up requesting to see THE GAME PLAN. I wouldn't have gotten that fateful appointment if Melanie hadn't tweeted me. I met the fab Gina Denny and Suzanne Gale via Twitter (we followed some of the same people).
DO stalk agents and publishers. There are amazing contests and chats via twitter that you will NOT see on Facebook. People hooking up with agents, finding out what agents are looking for, finding agents that are perfect for them. Isn't that, in and of itself, worth getting on twitter for?!?
Sure, okay, it's not for everyone and your writing career will probably survive if you just don't want to, but when I think of all the advantages, I know there's a reason I can't stay away!
So go forth! Tweet!