"We undercount active users whose accounts are protected, accounts that view tweets but don’t send any, and accounts that log in and engage in other ways beyond tweeting (like favoriting or adding profiles to lists)."

My markup. If I understand correctly, not having a public tweet is a marker for being a spam account. Isn't that kind of a lot of people? I know from other forums that there's a large ratio between lurkers and active posters.

Given that you need an account to customise your timeline, and, these days, pretty much for just reading a tweet, there may be loads of real reader accounts that never post and never bother customizing their profile.