Here's principle #7 from the practical guide to Gov 2.0 - Keep Users Connected
Social and web 2.0 approaches rely on continual participation. Continual personalized hooks and calls to action keep people coming back and keep people addicted. The trick is to let the application do the work. Think of the emails that you may receive from Linked In (Someone has added you to their LinkedIn Network) or FaceBook (Bob has sent you a new message on FaceBook).
Traditionally, within the community setting, a newsletter or similar tool was used to engage users on a weekly, biweekly or monthly basis. Applying the successful approach from social networking of continual updates throughout each day encourages ongoing interaction with the community, which keeps users both connected and engaged.