It's anecdotal but I remember being intrigued, not being able to get an invite, and then losing interest/forgetting about Orkut by the time it opened up. I think in general with invite-based launches there's a fine line between creating buzz and stifling it.
It worked well for Gmail, and it worked pretty well for Orkut at the start. For technical reasons, they weren't able to handle the huge influx of users that would have come from opening it up to everyone.
There have been some invite-only success stories to be sure. (You could almost count Facebook, too.) The usual way of handling it these days seems to be allowing people to add themselves to a waiting list as an alternative to being invited.
