Limiting the amount of people that can use it by making it invite only is stupid. They did that with wave and look how that went. A social network is just that. Social. What's the point in a "revolutionary new network" if none of your friends are on it because they didnt get invites?
It's not limited to make it popular, it's limited because it's not done yet. They're done with internal testing and are now doing a beta basically with "normal" people. Since they knew the press would get wind of this from anyone in the "beta" they just made the press a part of the beta so it could actually be covered properly. Once they actually get it to launch they've said they don't plan on limiting the number of people that can use it.