Google has had more than one product that I've been excited to use but was invite only, and then only got the invite after interest had died both for the public and myself. Ones I can remember are Orkut, Wave, and Ingress.
Making a social network like Google+ be invite-only when everybody was already on Facebook was an absolutely bone-headed move and I think was THE driving factor for its failure.
There were so many memes about people finally getting an invite to Google+ and then discovering nobody was there. Without your friends on it, you're not going to make it part of your daily routine, and eventually just forget about it entirely. By the time it wasn't invite-only anymore, the hype was gone and Google+ had already been considered to be a joke.