The short answer, in my experience is a flat no, social networks such as Facebook and LinkedIn are not worth the trouble, especially the time they take away from creativity. Consider them to be like the computer version of candy, fun and tasty, but the end result is cavities and obesity.
They are much like the comments section in the online newspapers, sure, you may be able to blow off some steam or voice a opinion or two, but that is all you get out of the activity. Nothing is accomplished, and for the writer nothing as gone toward finishing that manuscript.
Far better is the face to face interaction you can be involved in at your local writers group or one of the groups associated with your local bookstore, library, church, what have you. Going online and putting out your request for help to what could be a naked fat man living in his grandmother’s basement…ewww.
I’ve tried both, and though I do have both a Facebook account and one at LinkedIn, I’ve turned off the announcements, notification and email access those sites like to have into people’s private lives. You know what? My writing has improved and the page count has gone up exponentially.