The difference is that GMail is useful even if none of your friends are on it.

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.