Person A sends to B and C
Person B sends to C and D
etc.
But rest assured, the plan is that eventually you'll get a chance to reply to the people who sent you cards. I remember last time I was a bit keen, and also when it started looking like it was going to fold, just sent out cards to a lot of the people who sent me them anyway, but that shouldn't be necessary this time. Although since more people will get on board, things will get more complicated, and you may end up sending cards to the same person before completing a full round.
I sent mine yesterday
I'm considering basically reversing the sending order next round; so people immediatly respond to the people who sent them cards (we would need 2 more new people for it to work out as well). Do people think that's a good idea, or should we try to create as many links between people as possible, before responding?