Twitter should really allow advertising to your existing following.
I have an account for one of my projects that I use to announce new releases and updates. This happens once every few weeks, so I would pay for an option to stick my tweet at the top of my followers' feeds for, say, a week. If Twitter would then also allow people to unstick a tweet after reading it (or opt out of sticked tweets altogether), I think it'd be a great and welcome feature to have for everyone involved - TWTR gets paid, I get more eyes on my tweet and others won't miss my updates, which is what they followed me for to begin with.
This could really work! I think Twitter is much more about building an public identity than let's say Facebook. If they would allow their user to run minimal budget campaigns this could become much more successful/relevant than the typical Twitter ads.
They know when it's displayed to you. Let's say that Bob follows Sally (and a bunch of other people)
* Sally makes a tweet at 8AM
* Bob doesn't check Twitter until noon
* Bob doesn't scroll down, the tweet at the bottom of the screen is timestamped 11:30AM
In this scenario Twitter never displays Sally's tweet in Bob's timeline. It is thus not an impression. If Bob had scrolled down to see tweets going back to 8AM Twitter would display Sally's tweet in Bob's timeline, this would count as an impression.
If you're asking if they know that you actually read, parsed, comprehended, or contemplated the text of the tweet then no of course not. Likewise somebody clicking on a link doesn't mean they actually read, parsed, comprehended, or contemplated that content.
Of course, and that's how ad impressions are counted. But that doesn't help me as a user, and advertisers pay much less for "impressions" than for an ad that is interacted with in some way. So that's worse for everyone than an ad that you click to dismiss.
I have an account for one of my projects that I use to announce new releases and updates. This happens once every few weeks, so I would pay for an option to stick my tweet at the top of my followers' feeds for, say, a week. If Twitter would then also allow people to unstick a tweet after reading it (or opt out of sticked tweets altogether), I think it'd be a great and welcome feature to have for everyone involved - TWTR gets paid, I get more eyes on my tweet and others won't miss my updates, which is what they followed me for to begin with.