Someone else with some critique of Ello, much better written than I. This has had me stewing for the last day or so on social networks and needs. There's a theory of social justice I first heard referenced by Dean Spade, called "trickle-up social justice." The idea being that in focusing on rights and issues and support for the most vulnerable, and the least privileged, that you end up providing/expanding rights for all.
What would a social network look like, that started from this trickle-up idea. I.e. a system that started out with answering the question of how to provide a safe, controlled environment for the people who tend to be most vulnerable and/or targetted in online spaces. From conversations I've had, as well as followed online over the past few years, here's an off the cuff list of things that are bubbling for me:
controlled identity - you are who you present yourself as. Why is this important? Lots of reasons, I refer you to the nymwars from G+ and now facebook. This is something that Ello appears to be getting right at the moment (at least on some basic level), and is probably contributing to this rush of users courtesy of FB's current war on "not the name on your government issue id"-identified folks.
default privacy - when you sign up, you are unfindable, everything defaults to private.
granular access controls - you decide what pieces of information to provide to the site, to make visible, and to whom
controlled posting - public, private, specific group, specific person (imagine being able to take a triggering conversation with someone completely private between you and them without having to switch venues).
decentralized block-lists - host-provided abuse control seems to fail, constantly. Instead, allow people to create/curate block lists and share them (there are some 3rd party projects out there doing this today for twitter).
what about white lists - similar to above, allow communities of people to identify people that they trust, so new members aren't relying on just "figuring it out" (takes care of the "Oh, them, everyone knows their an idiot, just stay away from them."
revocation of content/permission
And I'm sure there's much more. For folks who want a social network that defaults to everything being open ("I want my profile public! I want all my posts public! Anyone can comment/reshare/message me!"), let that be something that they opt-into (even as early as signing up, a screen for the "what do you want public" options, where everything defaults to unchecked).
This may be a way that a system that prioritizes providing a safe and controlled environment could be usable by and accessible to a wide range of people and needs in a way that, say, Ello in its current incarnation seems unlikely to be.
It's an interesting exercise to think about things from this direction. Oh, for the time to play with turning something like that into reality. In the meantime, there are people who are far more eloquent, experienced,
 The context I saw this in focused on the gay marriage debate, and talking about how the focus on gay marriage benefited people who were already in a fairly privileged place in life, in a number of ways. If, instead, the rights work was focused on partner rights for say, poor immigrant queer folk, anything they were granted would benefit people all the way up the line in a way that simply providing gay marriage doesn't help those at the bottom.
 The reality is, there are many ways in which I'm privileged in my online social interactions. I believe this necessarily skews my take on the Things That Are Important™, that there are needs I'd be missing, or mis-prioritizing. if someone were to take on designing/deploying such a system, the Right Thing™ for them to do first would be to work with/hire people on the more vulnerable/targeted side of things, working out first hand what was necessary/wanted/desired.
 See also @aestetix's take
 No really, read this
 Ello defaults to all public, all the time right now. The company privacy thing is a good first step. I'm hopeful that Ello will pivot on this, but the reality is that right now, it is not there yet.