Why your Facebook feed is crammed with visual gags.

On a normal day, the Facebook page for NYC Pride, the organization that puts on gay and lesbian pride events in New York, gets about 10 new fans. A typical NYC Pride wall post gets fewer than 50 comments, and very few posts get any “reshares” (that’s when a Facebook user decides to broadcast one of NYC Pride’s posts to his full network). But on Monday, a world-changing news story forever altered NYC Pride’s social media presence: Kim Kardashian got divorced.

A short time later, Chris Frederick, managing director of Heritage of Pride (NYC Pride’s parent group), spotted an image on Facebook that summed up the gay rights message in Kardashian’s divorce. It was a picture of Kardashian and her former husband, Kris Humphries, on their wedding day. Overlaying the image was a pithy taunt: “If you think gay marriage cheapens the institution, two words … Kim Kardashian.” On Tuesday morning, Frederick posted the image to NYC Pride’s page.

By Thursday morning, the post had received nearly 18,000 Likes, about 2,400 comments, and—most significantly—almost 14,000 shares. According to All Facebook, a site that keeps track of Facebook fan pages, NYC Pride saw a huge spike in Facebook fans—over the course of two days, it earned 1,000 new followers. Frederick told me that he doesn’t know who created the image—he doesn’t remember which of his friends posted it—but that it proved more effective than anything else the group has ever posted.

NYC Pride’s Kardashian image is just one example of a trend that has overrun Facebook. Over the last few weeks, my News Feed has been inundated with cartoons or photos with smart comments attached. These images are often political—I suppose my network leans left, because several are pro-Occupy Wall Street and anti-Tea Party. I particularly enjoyed this riff on the “we are the 99 percent” meme.

The technical name for a picture with superimposed text is an “image macro,” and these graphics are an ancient phenomenon online. (Remember Lolcats?) But while image macros have long been ubiquitous on 4Chan, Reddit, and Tumblr, they were never really so popular on Facebook. Not too long ago, most of the images you saw when you logged on to your Facebook page were pictures of your friends, their friends, and various kids and dogs.

But in the past few months, something changed. Now images on Facebook are more political, they’re less intimate, and they’re more calculated—many seem designed to become viral hits, and many do become viral hits. This isn’t happening just on my personal feed; many of my Slate colleagues say they’ve also noticed this phenomenon, as did several of my followers on Twitter. I contacted Facebook to ask about image macros, but I got no response. More helpful were two people who follow the site very closely: Nick O’Neill, the founder of All Facebook, and Josh Constine, the lead writer of Inside Facebook. (After I spoke to him, Constine announced that he’s just taken a job at TechCrunch.)

O’Neill and Constine proposed several theories for why image macros have overtaken Facebook. Let’s review them:

Credit the social network’s new algorithm. As it seems to do once or twice a year, Facebook revamped its main page in September (as usual, people claimed to be unhappy with the changes, but they got used to them). The biggest change was to the news feed, which now features more recent content and gets updated with greater frequency. If you log in often, you rarely see the same status update twice. In other words, Facebook has lowered the bar for what appears in your feed. “Previously it was only the stories that got the most Likes or Shares or comments—or were posted by the user’s closest friends,” Constine says. But now a post with fewer Likes and Shares—and one that was posted by an acquaintance instead of a close friend—might make the feed as well.

Because this change favors stuff that was posted recently over stuff that’s more personal—and because personal content is less likely to go viral, since it’s relevant to a smaller number of people—Facebook has become a lot more receptive to posts that can be shared widely. Constine and O’Neill say other recent algorithm changes make Facebook better for spreading posts. The site now highlights stuff that several of your friends have shared (even if those people don’t know one another), and it posts a share count next to each item (and you’re likely to reshare something that has been shared often).

Taken together, these changes turn Facebook into a platform for viral hits. But that prompts another question: Even if Facebook is now a better host for viral messages, why have images with captions become the virus it’s most susceptible to?