1. Your users could have bounced and been removed due to a block on your server by a receiving server, such as AOL. If you had "notify when a user is removed for bouncing" enabled and a notification email address set up on the List Settings menu you should be notified of these.
Try changing the listing from "Active" to "Removed" on the User Database page. If there are a lot of entries here then users may have removed themselves or bounced.
2. The "Referring URL" is always recorded with ListMail. When someone signs up from a form the name of the web page containing the form will be stored. You could also use a custom field to identify the source of your subscriptions. Were you to use this, assuming your custom field is field #1, you would do something like this in your form:
<input type='hidden' name='user1' value='source1'>