An outstanding organic link is made on a subject appropriate, high traffic web page. Also, someone could make a backlink in a forum that would benefit you.
The social links are Facebook and Twitter. The context may have nothing to do with the subject of your hub, it's just that someone found your hub interesting.
I understand that you are rarely going to get income (clicks) from posting hubs on social sites. Also, Google seems to detect the difference and values organic links more, which can make your hubs rise higher in the SERPS.