Thesis: the key to success for any new social network is interoperability.
Google Plus's failure to thrive may not just be due to Google's real names policy, but also because it doesn't play nice with any services outside the Google ecosystem.
I can post my Flickr photos to Twitter. My tweets show up on my Facebook profile. My Instagram photos can be cross-posted to Tumblr. And so on. And with services like IFTTT, I can automate the process.
But Google Plus only integrates with other Google services. Auto-post new blog entries? You have to use Blogger. Easy sharing of photos? Better be using Picasa. Sure you can post links to your Flickr or 500px photos manually, but that's additional friction--one more step--and Facebook, Tumblr and Twitter will let you automate it.
When I post a new blog entry, my IFTTT recipes post it to Twitter (where it's picked up by Facebook) and Tumblr. Google Plus doesn't support IFTTT (or auto-importing non-Blogger RSS feeds, for that matter). If I want my blog entry to appear on Google Plus, I have to add it manually. That is, if I remember to do it.
Many of us don't use just one social network. We use them all, to varying degrees. The more they talk to each other, the easier it is to use them. A social network that closes itself off from other networks cuts itself off from the network effects of other networks.
So it'll be interesting to see whether Ello plays nice with other services. The Rules ban automated posting at present, but I wonder whether there will be room for a carefully managed API at some point, or whether Ello will keep itself apart on general principle.