Even behind the protective guise of the Internet, we still have to say "no" to people we don't know or wish we didn't, and we still have to keep it more professional than personal. What you can say on Facebook is not the the same.
Linked In has a wide menu of options to get the word out that you know what you're talking about in your field and you're open to discussions with others of like mind. As a blog reader and writer, I like to read comments, mostly political, until they get mean or silly. But when you post a query or idea on LI, you'll usually hear only from people who take you seriously and have an informed idea to pitch back to you.
I'm no expert, but every day I learn more about Linked In and it keeps getting better.