I agree that not everybody on social sites are 13 years old. Look at linkedin. It is very popular with professionals, and has been around a long time.
But if I had to pick one free method, and could not use another, I would have a hard time giving up article marketing and social bookmarks, but in the end I would probably go with the plethora of free blogs like blogger and wordpress you can put up to link back to your site.
If I had to pick two, I would probably use article marketing and free blogs. But I know that social networking works like a dream for others.
I think lots of things work, given effort and skill. So the thing that works best for you will probably depend upon your niche, skills, and personality.
I'm not sure what I just said about my social skills there.
