I'm assuming you work at ElloNo, I don't. Just one of the early (Sept., 2014) adopters.
The analysis I'm presenting of Ello activity, as my earlier G+ analysis, is based entirely on publicly visible characteristics and my inferences based on them.
I did join Ello during the genderqueer rush, though that's not a community I identify with. My comments at the time were along the lines of "artists and drag queens is as good a foundation community as any". Influence of the group as a whole seems to have leveled off a bit, though Ello's currently celebrating Pride, with a post on that topic from Paul Budnitz today:
https://ello.co/budnitz/post/mnwJqqER_ieTA6qe4YzV8w
And yes, a lot of notables grabbed their Ello seats, a few spoof accounts were set up (the Zuckerberg one may still be active).
I don't have any access to user numbers and haven't looked into building those, though the Ello crew have talked of growth and a few notable spurts. As I've noted elsewhere in this thread, I've seen both a lot of attrition of early accounts and new arrivals. I noted the same trend on G+ (and did a major housecleaning at one point of long-dead profiles from my circles there).
The FB comparison has far more to do with what that group represented than how large it was. Dynamics of online community growth (and decline) are fascinating -- I've been watching online communities develop, grow, and decline for nearly 30 years. Harvard students (or grads) are an attractive social cohort, it's an easy base to grow if you play your cards right. That was FB's strength. Now it's the overall size of the social graph, but they're vulnerable to defections and evaporative cooling effects given that, by definition, most members aren't high-social-status:
http://blog.bumblebeelabs.com/social-software-sundays-2-the-...
Colleges and creatives make good seeds, though they're not perfect. Balancing out traits within communities helps a lot -- e.g., geeks can be level-headed and realistic, but may lack enthusiasm or interpersonal engagement other groups can bring (I generally identify geek myself). Finding and cultivating the righ mix is an art, but it can be exceptionally powerful.