Touch every message record every time somebody creates an account?

Let's say you have 1000 posts, and 100 users. Worst case is the starting case: nobody has read anything, so 1,000 * 100 = 100,000 extraneous records must be stored until a user starts viewing. Add a single user, and you have to add 1,000 extra rows, even if that user only ever logs in once and never comes back. Add a single post, and that post has to include 100 extra rows to indicate the number of users that haven't viewed it.

With the viewing history boundaries, worst case: Mike's even-odd viewing, 50% of total number of posts, and will only grow as they continue to view even-odd.