Within 24 hours (or even right away), send them a personalized email asking them specifically why they signed up. What made them pull the trigger? This is extremely important data as you want to make sure that your product is delivering what your marketing messages are promising.

Ideally, with your analytics software you can save the note to their user profile. This is important so you can ask them why they canceled or chose not to continue and compare those answers.

Early on you can do this manually, and look to automate this (so long as it shows from you) later.