I think YouTube is overlooked as a social networking site. Thanks for mentioning it. I do like subscribing to my favorites there and even visit sites.
Answering questions is a great way to network. LinkedIn has a Questions area on a lot of different business topics so that is another great place depending on your business.
Hmmm, if I only had one, I might pick YouTube too, because there is so much there, music, book readings, funny entertainment etc.