I think the higher-ups at Facebook are just now realizing that they're facing their first real crisis. Diaspora likely gave them about a half-day of indigestion and then some good laughs. But Ello is the real deal when it comes to a threat.
That said, I predict it will not succeed as a Facebook replacement. Indeed, their founder insists that it's not intended to be such. Is that hipsterism? Probably. But he's probably also right. While they're getting 30k+ signups per hour, people are going to react like they did to Google+ - that is, they'll sign up, play a little, find that it has nothing that Facebook doesn't already have, and usage will drop off. Ello has significantly fewer features that people want. If Google+ didn't get traction, Ello won't, either.
Yes, people want to migrate from Facebook because of their policies, but this threat is likely going to be the catalyst that forces Facebook to back down on the real name issue.
For this reason, I think Facebook will weather this storm.
Now... want to know the issue that Ello could press that just might win it for them? Your feed. You don't want "top stories," you want everything, in order, without someone telling you what they think is relevant. You want to see it all and make that decision for yourself. That's Ello's concept of Friends/Noise and it makes sense. It's the one thing that Facebook won't back down about, and Ello could press this point.
Then again, Google+ didn't win that argument with "Circles." So maybe that won't work after all. But I think Circles were before the relevance issue came to a head.
Time will tell. But at least I'm on record so I can say I called it ;)