Facebook is already copying the features that make G+ cool/useful (circles, etc), and the one big, inherent advantage you had in your favor (Your user base, e.g every Google account) you alienated by keeping it "invite only" for too long.
Either open it to everyone sooner, or hold off on this prolonged "invite period" so you don't risk confusing / alienating people that want to try G+ but can't.
G+ feels like its the same 10 guys posting the same thing over and over, while Facebook feels like, well.. Facebook.