If you have customers -- not users -- with accounts in your system and personal data attached to them that you want to track, manage, message, etc, you may want to consider setting up a Customer Relationship Management system.

There are a lot of companies in this field and yes, it can get expensive. Some of the top players are Salesforce, Sage SalesLogix, SugarCRM, Netsuite, Oracle, Microsoft, and other. You can start by checking out this list of reviews.