Can't it just be a hash of the URI?
Unfortunately, that would often break. We've put a lot of effort into URL normalization (ie, http://smile.amazon.com/TP-Link-TL-WA701ND-150Mbps-Wireless-... is the same as http://www.amazon.com/dp/B002YETVVE/). When we can do "good enough" on the client so we can provide the same benefits, we'll do it. Or: provide a setting that forces URLs to exactly match, and we can use a bloom filter to keep traffic down.
Hear you loud and clear. We're working on a strategy so that it's not needed. Right now, that information is used only to determine if you or a friend has kept a page, or if you have previous conversations on the page.