In a social network, there isn't an use case "post text and pictures" - the real use case is "post a picture of my kid so that my mom can see it"; and Diaspora fails at that, as my mom isn't there, she's on facebook. An useful use case is "post a morning rant so my friends can see it" and it fails if my friends aren't on that network.
"read user posts" is a weak feature, but "read Bieber's posts" is a feature greatly valued by millions of teenagers - and Diaspora doesn't have that feature, Twitter has.
The core value of any social site or network isn't in the software, but in the community. If the interesting people aren't there, then the service is useless to me, so I won't read or post. If the facebook site was opensourced and used by Diaspora, even then it would bring orders of magnitude less value to users simply because of the smaller community.
What is Diaspora's plan for solving the user acquisition? Facebook replaced university 'facebooks' to get initial 'hot' users; Twitter got the critical mass from a few early adopter celebrities; Google+ is bootstrapping off of gmail contacts. Tech part is 1% of the resulting value, user part is 99% of it.
If Diaspora can't show a reasonable path how they will resolve the chicken-and-egg issue to get a critical mass of users, then it's not worth to invest time and emotions in posting there.