Even though I failed, I benefited quite a bit by taking on something so ambitious, and whether Diaspora succeeds or fails, the people working on it will most like be able to say the same.
Didn't it occur to you that even if diaspora goes nowhere, the work these guys have been doing may be influential or even the foundation for something that will actually go somewhere in the future?
While "only" 4 years, Muhammad Ali sacrificed what would have been the prime of his boxing career as a conscientious objector. He was famous as a great athlete before, but transcended it after.
Alexander Calder was an engineer for about ten years before becoming one of the great artists of the 20th century.
Many other examples.
Any way, the goal is not to kill facebook. It is to experiment how a decentralized social network could look like, and provide a solid alternative to those who don't want to see their data analyzed. It's nothing more than that at the moment.
For the first point, our protocol is improving (see https://blog.diasporafoundation.org/43-our-federation-protoc...) and is the only one really production ready imo, and for the second point, the user experience is getting better at each release, with new features.