I'm constantly amazed at how many start-ups ignore the community.
Do you win if you develop the best platform?
You win if you develop the best community.
Reddit, Facebook, LinkedIn, Craigslist weren't the best platforms (they've gotten much better). They were the best communities. They've grown and expanded based upon the strength of their community (and become platforms for communities).
Great platforms don't attract great communities. Good community building attracts great communities.
If you get the right people, have them do the right things, and foster a sense of community, you will succeed.
That means you need to target the right people to begin with. You need to know what you want them to do and their motives for doing it. You need to incorporate sense of community elements into your start-up activity.
My bet is if more money was spent on community development, more start-ups would be thriving.