Social Media: Places for Engineers to Hook Up?

Engineers once devoted several days per year to attending professional conferences and seminars to meet with their peers, build relationships, and exchange information. However, due to slashed travel budgets and other factors, much professional networking now occurs via online technology/social sites like GlobalSpec's CR4 and social media (SM) sites. Other than LinkedIn, where most engineers have a profile posted and are able to network for job opportunities or to perhaps locate old college chums, what other SM sites do you find useful?

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