2016-10-22 UTC
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KartikPrabhu then spec also says "As in almost all pubsub systems, the publisher is unaware of the subscribers, if any" but https://indieweb.org/How_to_publish_and_consume_PubSubHubbub says about Superfeedr "If you register your own hub at Superfeedr, you will also get access to information such as the subscribers to each of your topic URLs"