The number of registered users was sitting at around the 2 million mark at the end of 2014. Since then, they've started issuing invites for people who asked for them at a more rapid pace (one Ello developer said signups are now at a rate of around 250,000 a week). Of course, there's no way to obtain an indication of the number of active developers from this.

I'd speculate that the reason they held off issuing solicited invitations is that they spent most of December 2014 performing infrastructure and backend upgrades and improvements. (The fact that these upgrades happened is fact, the connection with the change in invite policy is speculation).