I'm in my mid-20's and use social networking sites to meet other young professionals, keep a dialogue with old friends from grade school/high school/college, share photos and meet groups of like-minded people. You have good question, and I realized there aren't many older than 30ish professionals that I know in the "social-sphere"... so a quick online search led me to this entertaining article: news.bbc.co.uk/1/hi/technology/6690569.stm

LinkedIn is a great way to network within and without the industry. If contact settings include 'inquiries', you can call that person and get answers as opposed to the typical 'cold call' response. The Answers/Questions allows you to survey others in a given segment regarding their experience. It's also a good tool to stay in touch with colleagues, since they update their profiles as circumstances change.

I use LinkedIn to keep up with former colleagues who may be looking to change companies - I have yet to use it to promote my company. It seems to be very popular with folks who are looking for employment. PSDA started a LinkedIn group, but moved it to printjunkie.com