Big social networks have this feature. 

Have you seen mail alerts that networks like Facebook & Linked in send to their users periodically ? This feature will send my users periodic email alerts of whats happening on your site or social network. Users will be able to select if they want to receive weekly, fortnightly or monthly updates. 

For example mail may include:

• User's friends' activities
• How many user visited your profile after last login
• How many unread messages since last login
• Friend suggestions
• Latest groups/users/articles from some sections
• Pending friend requests
• Latest activities from your groups

Its a great way to keep your users in touch with whats going on on our websites and keep them coming back. 

Thanks.