There is also the concept of partial consumption. Often you might click on something you think might be interesting, peruse or preview it, then decide it wasn't as interesting as you thought and move on. Did you actually consume that content or did you preview it? There is no way to know if someone is lurking, actually fully consuming the content or simply previewing it. There is probably no way to prove this but my guess is that 90% of the consumers are actually previewers.