I'm interested to know how they have been perceived as failure. I think this is all very relative, and failure is a strong word. Github still have 1 day old commit and the community is apparently still active.
* Source-code full of freshman's mistakes and security exploits. Basically not-to-be-trusted quality code, released to the public for deployment.
* All money invested in the project wasted without anything to show for.
* I know absolutely nobody using diaspora, expect one person.
* This person attempted to invite me, and diaspora failed to send me the invite email. Now he cannot invite me again.
It's a laughing stock. For the parts of the internet which has even heard about it. Which is the minority of the internet. As a social network this is a failure on absolutely every aspect I can find measures for.
The goals were admirable, but goals alone wont win you any credit. You have to get there as well. Diaspora didn't and most likely wont.