The invite model worked well with GMail, because once you joined to it you could contact any one, after all, is just mail.
But with social networks it doesn't work the same, If you need to send invites, explain to them what is axactly an invite, then explain to them how to send invite if they want to tell their friends, it is to much work just to join.
The number of Facebook app invites I get daily tells me the average social network user understands the concept of invites. It's another step to get them to sign up for a social network, but I've already had people asking me for invites, and they follow through to the signup.
In your case, but in mine, my circle of friends are just computer casual users, and Facebook invites are not mandatory to join, you can go to www.facebook.com and join there, no invite required, you can tell your Friends to go there and join and not to "hey, waith till I send you an invite", and that's another issue.
