I think the bottom left quadrant should be rounded: if a website doesn't scale at all, there's no way it can have a lot of users. I wonder if there are other constraints like this one, and what shape the graph would then take...
Good point, that's probably what happened to Friendster. The site was so slow that users stopped coming back. If you have lots of users and your site doesn't scale, they will check the site less often or abandon it completely.