Recently, I saw Steve Rubel tweet that he didn't think Pinterest was particularly viable compared to some other startups he's seen.
My reply was that Pinterest will succeed because it affects my life and the life of my friends offline. One friend who's a teacher used an idea from Pinterest to teach her class about the different states of matter. I've used a few ideas from there to do my hair and make games for my kids to play.
Pinterest crosses the barrier between digital and real life. Why is that important? Because most people do not live the majority of their lives virtually. And I posit that any online startup which becomes a part of our real, non-virtual lives has more staying power than one which can simply vanish into the ether as if it never existed.