Getting Gmail Account was something nice back in around 2005: for me.
One of my friend got Gmail by invitation and I was kinda jumping into it.
Now since Gmail has been beta (forever: ha ha) and have been made available freely. One does not need any invitation.
So, invitation feature in the gmail looks like vestigial organ.
Vestigial organs are those organs which found its use in previous stage of evolution but are no longer being used.
So,Invite a friend feature should be discarded.
Google! Just a suggestion.
I know you have a lot of smart guys in there.