I think most look at the ideas and don't see a safe/profitable path. Most are right but with the right person they can take any idea and make it work. Not because of the big idea but by winning the small battles and the non-obvious battles.

Facebook had a similiar idea to others. But they limited it to Harvard. So anyone who joined was part of a social club that they saw in real life. That gave them trust and people more freely posted. Next they rolled out to only ivy league schools. Next all colleges. Then highschools. When they released to the public the colleges still had semi-private networks allowing them more freedom. Then facebook did something no one thought was possible, they allowed you to login to hotmail import your contacts. Now you are friends with people you may not have talk to in years.

The original idea only shaped a broadvision. Each decision really provided the vision and a path for growth.

The original vision is important. If you tried to take a dating app and follow the Harvard facebook roll-out it wouldn't work because people wouldn't want to be seen as looking for a mate. Dating sites try to connect strangers.