An RSS or Atom feed is a live document of recent events or items. For example, there are feeds of Twitter tweets, Facebook posts, and YouTube videos. Usually, other programs track the feeds, and then let you know when there's something new.
You can do other things with feeds, too. I've collated a few of my personal feeds into a Lifestream that updates automatically. It's a very convenient hands-off diary.
Companies like Twitter and Facebook have been trying to figure out if they should monetize feeds. They already inject ads into their main user interfaces (into Facebook's users' walls, and onto the Twitter timeline, for example), but what should they do about the feeds for the same information? So far, they've left feeds alone.
This week, Delicious tried injecting ads into its users' bookmarks feeds. The bookmarks feeds are activity feeds, because they reflect actions that the users have taken. On Delicious, users bookmark sites that they find noteworthy. So these new ads are clearly marked as Sponsored items, because otherwise it would look like everyone suddenly explicitly bookmarked the ad's product. That'd be straight up deception.
Still, there are problems with the way that Delicious is embedding ads into their users' activity feeds. To ensure that the ads are at the top of the list, they're always coded with a pubDate ("Published Date") of the current time. This goes against the original intent of these feeds, where each item's pubDate doesn't change if there's no actual change to the item itself. Old ads should get old, too.
Instead what happens is that every time the feed is fetched, since there's always a "new" item (the same old ad, but with a new, just-now pubDate), is that Delicious can no longer return HTTP code 304, which means, "I'm not providing the feed again, it hasn't changed since the last time you got it."
This is causing Delicious to be slammed with having to deliver full feeds for every request, and for all the clients to have to process what look like brand-new feeds. Beyond that, people who auto-tweet what they bookmark are all inadvertently tweeting ads now.
How did I discover this in the first place? My Lifestream started journalling that I've been repeatedly bookmarking the same ad over and over again.
That's not the way to monetize activity feeds.