A good question indeed. I suppose it depends on what you are trying to accomplice/get out by having a presence on these sites. They certainly open doors to "increase exposure, share ideas, and market a product", not to mention create contacts that might be valuable down the line, but how can one truly know where to draw the line between social media productivity, and aimless wandering.
Given the countless hours we spend optimizing our presence on each of these sites, and the frequency with which new ones keep popping up, is it best to stick to the sites we have already established ourselves on and keep optimizing until something worthwhile materializes, or do we jump on the train of whats popular at the moment, and hope to pick up little pieces at a time.
It's amazing how many people are oblivious to this simple task. I have had to take this issue up with several companies who send out newsletters or promotional materials and expose hundreds of email addresses.
Definitely placing the link in my signature for a while.