What would be a possible use of this attack? I can't think of anything useful you'd do with knowing that you've visited Facebook. And so many people use sites like Facebook you might get a better success rate just always returning "visited" rather than measuring this way!
Maybe you could use it to only show those social sharing widgets that are for services the visitor actually uses. Though I guess WebIntents will eventually be a better way to handle that.
Ad retargeting without going through one of the high-reach ad networks. Or any product site instantly knowing which competitors you've researched, and tailoring their pricing to that.
