I've found LinkedIn really useful, more than from an SEO point of view, because it brings me in touch with people in my field (mechnical engineering).
Twitter, Facebook, Google+ and Linkedin are very much helpful not only to increase traffic but also to increase brand awareness and if you use it in a proper manner then you can generate sales through such potential platforms. These all are best way to reach the mass with a single click but for that you need to build a strong network.
Without strong network these all tools are useless.
Not only Twitter, Facebook, Google+, Linkedin . I think All Social Networking is important for increasing Traffic. And forum is also like a Social Network.
Since this thread is so tired it has begun repeating itself, it's time for bed.