Social networking sites aren't just for sharing your thoughts and photos. They also provide a soapbox to say "Hey, look at this cool thing I found!" to everyone you know, all at once. But is there a proper procedure you should follow when you share things? I argue yes.
This particular etiquette topic feels extra pedantic to me, because I'm in the content producing business and try my best to properly credit whenever I can, but I know it's very much a "who cares" topic for a lot of people. Forgive me if this feels heavy on the navel gazing.
The golden rule applies here. When you find a news story that you share, you would like a nod in return when one of your followers or friends re-shares it with his followers. You didn't write the news story, but you found it first, which means you should get a little credit for your eagle-eyed ability to keep on top of the internet.
As for images, video, comics and other art, you should always credit the creator when possible. This is sometimes difficult to do, because a lot of the times the place where you found it — Tumblr, Twitter, etc — did not. But just think about what happens when you see someone post something you like; you want to see more of it. You want to see what else the creator did and find out how they did it. Try not to deprive your friends or followers of that.
But by far the most egregious etiquette violation is taking credit for someone's original witty remark, quote, or joke. When you see something great on Google+, use the Share button — don't copy and paste it into your own new post. There's a Share button for a reason! When you're on Twitter, use the Retweet link (or do a RT) so everyone knows who the original author is. It's even less work than copying and pasting.
On Facebook... well, Facebook stinks and there's no built-in way to reproduce someone's post while giving them credit, so do what you can and at least link to the person's comment when you re-share it.
These principles are even more important when blogging, because unlike social networking, it has a feel of being a bit more formal. You've got a bit more time and are expected to craft and mould your message a bit before posting, meaning proper sourcing makes you seem like you know what you're doing and expect other people to do the same for things they take from you. You can group sharing sites like Reddit and Digg in this category as well.
Again, this whole topic is pedantic to the extreme, but it boils down to one simple rule. Whenever you're posting something on a social network site, just think think about how you would feel if you come up with something and someone takes credit for it. Not good, I would imagine.
Etiquette is, by its very nature, subjective. What's acceptable today probably wasn't acceptable 20 years ago, and what's unheard of right now might be commonplace in a decade. It might also be seen as pedantic to some, because if you boil it down, it's just an arbitrary set of rules that we all abide by to make all our lives easier. So weigh in and tell us what you think about this subject. It's one of the few times that just about everybody's opinion is more or less as valid as everybody else's.