I use the basic services of LinkedIn to stay in contact with former colleagues in the IT business. That can help with finding contracting gigs and just keeping my name in the ring.
So far I’ve not felt a need to explore FaceBook or MySpace.
I spend about an hour per month on Stumbleupon but no more than that because my time is more profitably spent elsewhere. (Stumbling sites is enjoyable but also addictive!)
Do you have a story to share about social networks and career change?
(2012 March Update: I still use LinkedIn but am also active on Twitter and Google Plus.)
(2018 Jan Update: I deleted my LinkedIn profile in a fit of pique haha. I use G plus to post links to new blog posts. Starting to get more active on Twitter (@markmcclure2day) this year as part of my SF writer persona.)
(2020 Update: G plus gone now too.)