I asked them about that (http://techcrunch.com/2016/01/19/trello-launches-developer-p...). Those are total signups, not active users. They are not disclosing monthly actives.

It's not really a secret: https://www.quantcast.com/trello.com

Yes it is. Some users will only ever use the mobile app. I don't know anyone on my teams except myself who log in on the web.

Quantcast has our mobile data too.