How can I track statistics regarding the number of sign-ups vs. account deletions? I'm working on a social networking site in Drupal 7 that I haven't launched yet.  However, once I've launched I would like some basic metrics to track how I'm doing, particularly the number of sign-ups per week (or some other arbitrary interval) and the number of users who have cancelled/deleted their accounts.

Is there a way to track this from within Drupal?
Views is your friend. You can build a view that shows the total number of registrations per week, for example. You can use an exposed filter to change the date ranges.

You can use user stats to keep track of last login time and then built a similar view. 

If you user userpoints you can get stats based on activity (or the lack thereof).