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.<p>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).