Email is the obvious one. With their email, you can get back in touch with them and pull them back into the site after they (inevitably) drift off.

Email is the bit that sticks... without email capture, there is no stickiness. Without stickiness, growth is extremely difficult. You would need to make a site so compelling that people come back to it even without being reminded. That's really, really hard (think "building the next google" kind of hard).