I think they should have given every new account exactly two invites, and see how quickly it grew from there. Then they probably wouldn't have to shut it down. That kind of artificial scarcity might have made the invite process a bit more fun, too.
One invite a day would be more sustainable and get people to actually invite others.
If I had only two total, I could give it to folks who might ignore the invite, thus crippling my network (I would more likely end up hacking it by making numerous fake accounts and harvesting the combined invites... not that organic or desirable for google).
