The feature was announced in the run-up to Macworld Expo, San Francisco. It was operational for a short period, but a bug made data unstable, Apple said.
Subscribers to .Mac can deposit data online, where it can be accessed by that member from any Web-connected computer. Apple's iSync can be used to keep that data up-to-date both ways - updates to the Desktop Address Book can be uploaded to the Web-based version, and vice versa.
The online Address Book lets users maintain email addresses, phone numbers and addresses. It also offers a Quick Address feature that lets users send an email to up to fifty recipients with a few clicks.
Users need a valid .Mac account and the current version of iSync in order to use the facility.