I like GMail because I get a quick-contacts list, lots of space, filters & labels, and the spam blocking is just plain spiffy. Also the conversation-view is handy.
I have 100 invites, and I've sent out a total of around ten, I think. I think the number self-increments every so often (per day?), so you'd have to be sending them out pretty consistently to run your number down to zero.