At my non writing job we had to take a couple classes on Social Media and while I found I knew most of the information, I realized that not everything is quite as obvious. It may feel like second nature to me, but for others (like the classroom around me) it might not be. So I took notes on all the important stuff and assembled it below, in hopes that it might actually help someone else out there. This is stuff that applies mostly to promoting yourself and your business on Social Media, but can apply to normal life too. (Keep in mind I'm not an expert, this is just my notes from what I learned).
The most important things to keep in mind when presenting yourself on social media, from a business perspective are Honesty, Transparency and Authenticity. It boils down to don't lie on Social Media. If you lie there's a record out there of what you said (or that you deleted it) and you will be found out. So it's generally just not a good idea.
Have a disclaimer
When you're creating a social media site, blog or anything like that, especially where you're going to post about entities and properties other then you're own, make sure you have a disclaimer (usually in your profile) that all of your views are your own.
When you see a link you like, don't just reblog it. Add your own thought. Keep in mind that if you and those following you have overlap in friends, they'll see a link several times with the same information. What's the point of them looking at your post as opposed to everyone else's? This is especially true with things like Twitter and Tumblr. Instead, you should add a hook, something extra to catch their attention and hopefully steer them toward you and your thoughts, along with the link.
Also, in the same vein, when you reblog or retweet something, make sure it doesn't cut it off. For example Twitter has a limit of characters, but blogs and tumblr don't, so sharing a link or post between them might cut off. Be mindful of this and edit if needed to keep it looking clean and professional when sharing information between sites.
Avoid autoreplies. They're boring, they're mechanical and no one really wants to see them, especially if it's a shameless plug like 'buy my book'. Either respond personally or just don't do it. It cuts down on clutter.
Credit Your Sources
A Golden Rule of Social Media... if you take from a source, credit it. This applies to writing, to pictures, everything. While the internet is hugely public, you should also respect the others there. If you use an image, it doesn't take much to say where you got it. Same with news and everything else. Credit your sources, it's just plain nice.
Don't be a Troll
Don't be a troll. While it might feel awesome every once in a while to poke at people and fan a flame, it doesn't look good for you. Very few people can develop an online persona that's loved by being a jerk, so just don't do it. You'll drive away more people then you'll gain and that doesn't help.
Don't over share, this is a business version of you if you're using it for promotion. Over-sharing drives people away and can make people feel uncomfortable.
Write for scanability. Use headlines and break information up so it's easy for people to scan over and read, even super fast.
Finally, don't get overwhelmed with social media. You don't have to update it all the time, every second of the day. Start simple with once a week and then increase as you feel comfortable.
What tips and tricks have you come across that apply to social media advertising and plugging as a whole?