Another 2c: you should think about publishing numbers/stats for annotations. Easiest to start on the level of namespaces. Publish stats about popularity of namespaces: how many tweets and how many users use which namespaces. And don't do "that's a good idea and there are still many moving parts and we are thinking of it for the future," do "this is absolutely vital for the community from day 1" :) This would be a good measure for community to inform what namespaces to support, what works and what doesn't, etc.
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