http://xing.com http://xing.com does it pretty well... Perhaps more or less like this: Build a notification-queue for each user; store the time of the first it

http://xing.com http://xing.com does it pretty well... Perhaps more or less like this: Build a notification-queue for each user; store the time of the first item; every x check the queues if the first item is older than x send all items as a batch and flush the queue.