As with all things Automatable the hard part here is preventing motivated, skillful individuals from gaming the system. A free car is probably sufficient motivation.

Any attempt to create some kind of “subscribe and click” bot would probably leave detectable behavioral and/or other data patterns if one were willing to invest in systemic intelligence to detect them and you’re quickly into an arms-race. The other option is to just make make signup annoying enough but this drives down your conversion of legit audience oriented humans.

All of this would increase the cost of acquisition (but maybe you have a product at the end?)

Perhaps Facebook is the way to go, you gain additional insight into the user (via their graph) but I can build a bot network in Facebook — perhaps Facebook is good at catching and dismantling these but I see no evidence that any of the one-off accounts I’ve ever created for development purposes has ever been deactivated. Of course none of those accounts are doing anything but lying dormant: nothing to see here these are not the droids you’re looking for. So perhaps you could force ‘sharing to my network’ which might be enough activity to trigger FB bot detection? Once again you push down your audience conversion.

Interesting problem. :)