Even though I moderate around 15 gorups I rarely go to the front page for a look and could only imagine that those members who get their posting via email (as opposed to the website) would rarely see it as well. If I had not mentioned the new photo, I wonder what percentage of members would know about it a week later. Truth is it would be very few and mostly those who chose to read directly off the web.
Few people probably go there to read messages, but one needs to go there to see new entries into the Files and Photos sections. The extent to which such new entries arise is of course very different across groups.
