Writing for Facebook and Twitter isn't that different from other web writing. It's likened to micro blogging.
Writing a tweet is similar to writing a good headline - action words, leave out the articles (a, an, the) and start with the specific key words.
LinkedIn is more B2B, where as Facebook is more B2C when it comes to companies using it.
Using Facebook and Twitter for marketing always has to be for the customer, not the company. The customer is always listening to WiiFM (what's in it for me?).
So, tweets and wall posts always have to provide a benefit for the customer or provide for their needs not be marketing the latest product or service for the company.
You can get away with that occasionally provided there's lots of other stuff for the customer.
If you're not already on Twitter, I'd suggest signing up and searching for "social media" for people to follow and learn from them. Follow me on Twitter @jason_clements
Copyblogger (http://www.copyblogger.com/) is a good place to learn about writing titles and blog posts.
Have any other tips on writing for Twitter and Facebook?