It should absolutely be off by default. The way these things spread is one tweet that is your message, then the next tweet that says "sorry I didn't mean to pos

It should absolutely be off by default. The way these things spread is one tweet that is your message, then the next tweet that says "sorry I didn't mean to post that". Just make a button or a checkbox that says "Tweet this" and lots of people will do it. The behaviour should be opt-in, just like email subscriptions.