Recent quotes:

Charlie Warzel at Buzzfeed reported on a study from Stanford (Quantifying the Invisible Audience in Social Networks, Michael S. Bernstein, et al) that says that we all underestimate the size of our ‘audience’, by which the authors mean the number of followers that actually read what we post, and underestimate by a factor of four.
maybe it comes as no surprise that people don’t expect to know how many actually read our tweets
Offline, there is no way to know how much others care about what we say. With our close friends, we expect feedback: we mention something about our day at work to a friend at a bar, and he responds with his own tale of woe. But when we say something in a group of 10 — say in a business meeting — there is no norm that leads to us finding out if all the folks in the room are thinking through what we said, or paid attention to it at all. In real life we accept that social indeterminacy all the time.