So I love Courier and minimal design and I think Ello is pretty beautiful in the whole hipster cargo collective tumblr VSCO minimal aesthetic. I mean, look at my tumblr, I've been using a similar theme for a while: http://blog.lorenbaxter.com
That being said, this doesn't yet even come close to "replacing" Facebook. Facebook's platform is super broad and makes a lot of things easier. There's plenty to complain about, but Ello has many thousands of hours of work to do to compete. Events, photo albums, groups, mobile apps, realtime messaging, group messaging, identity platform / global authentication, the newsfeed, deep integration into iOS & Android... These features are useful, well made, and pervasive. Not all of them are necessary for a strong social network, but together they make Facebook extremely compelling.
The media loves to make things really black and white, so of course the story is that "Facebook users are jumping ship for Ello." I personally don't know anyone who has stopped using FB for this and doubt that will happen any time soon. Perhaps a more realistic story is that Ello is a different place to hang out. A place where, hopefully, people feel more comfortable sharing personal things, or different things, or to have a different kind of conversation. But it is as much of a "replacement" for Facebook as Twitter or Instagram is. Which is to say that it's not.