if Ello were a disease what would it be? Seems like the common cold to me, highly infectious, but majority will recover within a few weeks. Most people sign up, can't find their friends, have an empty newsfeed and nothing to do apart from clicking around trying to find their friends. Not a particularly useful way to spend your time. The only point of interaction is the occasional cough when a friend follows you and you get an email notification. After a couple weeks most people will start ignoring these emails and their recovery will be complete.
Some people like myself have weaker immune systems to social networks and will have the patience to stick around for longer and find friends who are posting content. If you compare Ello to Instagram, the majority of the first Instagram users had at least one post and there were multiple ways to find your friends from Facebook to iPhone Contacts integration. Instagram looked more like herpes, a permanent problem that would become infections every time you posted a photo to Facebook and Twitter with a nice filter on it.
Now that doesn't mean the Ello can't mutate. If they build some way to help people find their friends people would take much longer to recover. I'm guessing Ello wouldn't use Facebook or Twitter as they want to be independent, they don't have an iOS app so no contacts integration, but a LinkedIn style address book importer and/or algorithm to suggest friends based on who invited you could work.
Ello's greatest hope is the host population who have clearly already become chronically infected. Whilst most people will recover quickly the disease can survive in the host population for long enough for the product to mutate to a point where it could re-infect those who have already recovered from their first exposure and next time they may not recover so quickly!