I'm even worse. I created a Twitter account a few years ago and immediately asked myself "why am I doing this?"

Logged out without sending a single tweet and never went back. I still get a few emails from them every month wondering what happened to me. Still can't figure out why I would need an account.

I used mine once to enter into to some contest. It's useful for doing that.

It's the new toss your business card into the bowl thing.

Yes, that's exactly my situation. On the outside it looks like you are not active, but actually you are regularly on twitter.

Unfortunately, there is no way to measure this case, except for counting the people you follow or the people who are following you. Then you could know whether that user is somewhat active.

Couldn't Twitter track how many times you "request" your feed? Although not perfect, I'm sure they could get an approximation of those users who are just "lurking". They could also track logins from the web side, and probably app usage.

This data wouldn't show on outside measurements (like in the article) but would help Twitter know who is active and not.

Twitter definitely could, and they can easily get a clear idea of how many of these 'zero tweet' accounts actually are active. However, Twitter have no reason to give this information out and there are not many ways for outsiders to discover this information.

There would be many business reasons to not release such information.

Exactly, doesn't matter if someone tweets or not. Its the active user that matters..