One of the tricks I've learned over the years is when a new user doesn't confirm their account via the Confirmation Email, it may be as simple as them having typed in their email incorrectly. (i.e. newuser@yaho.com; newuser@gmial.com; etc.)
When coupled with @AndyB's New Members add-on (where you can see just who is still in an 'email_confirm' status), the Resend Activation Mail add-on makes it super fast-n-easy to do so once you've corrected an email-address error you've tripped across.