The immediate advantage of joining an online social network, is to see who other people say they are, by reading their homepages. The best networks have discussion areas where people who share interests can exchange views. Any member can read those views. Sometimes (on Ryze for instance) these discussions are visible to anyone on the web.
There are two things you can learn. What other people are thinking about; and which people have ideas and points of view worthy of your time and interest. Join groups that talk about the things that interest you. Tell the people you admire how much you appreciate what they write.
The first problem people strike, is the need to explain who you are, to fill in your personal profile. Most people stumble on that step. You just need the courage to say something simple and easily understandable. If you need to dress up your profile a little, add some quotes from people you admire.
Do a lot of reading. Find out what's going on. It's essential that you find the courage to talk to or write to these people or to the network, for two reasons. Firstly so that other people on the network begin to "see" you. If you remain silent you are essentially invisible. More importantly so you become directly involved in doing your own thinking and responding to what other people have to say. I can't stress strongly enough how much difference that makes. It turns you from an inactive member into an active member.