Behavioral economics. You're more likely to invite 3 people if you have 1000 invitations than if you have 3 invitations. There's less to lose. So you'll invite the 10 people you know who would enjoy a game like this and 3 will join. Win win.
One of them involves changing a single number, the other involves creating new functionality. It's much easier to increase the number of Facebook invites someone can send out than implement a new invite system. The devs have said repeatedly that email invites are coming. I'm sure they'll get implemented, and it's not like upping the Facebook limit will prevent them from adding email invites.
When? I know you could sign up with your email and eventually get an invite. That still exists. But I haven't been aware of any way to bump your friends higher in the queue with email, and I've been playing since mid-alpha.
When the "queue jump" is open, anyone who has previously entered their email address in the "I'd like to be invited" database, can click the "queue jump" link and ask to get an invite right away.
TS sends an invite to that email address. The invitee clicks a link in the email and is taken to the log in screen to create a new account. That's how I got mine; that's how I got my friend an invite yesterday. Nothing involved except email.