Indeed. From the perspective of a learning algorithm, what would it think about either of these links? They've both been shared on Twitter, both been retweeted at least once on Twitter, and one has been curated here on Hacker News.

Alas, ultimately actions taken by people can always been simulated by spammers. We talked about this a bit on Fred's post: how do you know the act of sharing is legitimate and not spam? Hacker News employs votes, but that data isn't easily extracted by a learning algorithm in a general sense. I can only assume that part of solving this problem in totality is coming up with a way of determining the realness of the human curator. Otherwise, spammers will attack as soon as it gains any sort of momentum.