> I'd like to see a social network that limits the number of people you can interact with to ~100. Want to add a new person? Then get rid of an old one.

You can do this on your own if you so care - I use Twitter in pretty close to this way (I follow 158 accounts as of today, and ~10 of them are service status accounts). Anything over that I might also be interested goes on a List and I check them whenever I'm bored (and I also cap lists at ~150 or so accounts, but none of them are even close to that).

I'm of the opinion that software shouldn't build limitations, it should only be a tool for how users want to use it. (It could be a better tool by instead telling users "Hey, you're following a lot of users, things are probably pretty noisy for you huh? Maybe you want to use a list for {these similar accounts} or something?")