Your college roommate has been to 37 of these 50 United States. Your sister-in-law has tried 22 of the top 100 beers. Your coworker has tasted 94 of these 100 exotic foods. I can’t log into Facebook without seeing it in my feed.

These “challenge” apps on Facebook have gone fully viral — reaching 13 million users since the first one, The BBC Book List Challenge, was quietly distributed to a few friends in December.

They’re the work of Eli Dragen, a lone developer working out of his home in Lawrence, Kansas. Thanks to ads, his challenge apps make enough money to support the four servers required to handle their traffic. The first one took him a week; he estimates he’s spent a couple hundred hours over the past few months on the others, including travel, food, beer, books, video games, and movies and music.

They’re not particularly pretty, and they’re mostly packed with stock photos. They’re incredibly simple, and once you’ve done a list and compared yourself with your friends, there is no real pull for you to revisit and update your list after trying a new food or visiting a new country.

But the appeal is undeniable: It satiates curiosity — how adventurous am I? — as well as that subtle self-promotion inherent to social networks. Hey friends, look how many of these fancy beers I’ve tried. My tastes are exquisite!

And did I mention, 13 million users in a matter of a few months?

Dragen started in December last year after a friend tagged him in a meme called the BBC Book List Challenge. He was to copy and paste the list of books that BBC had apparently claimed most people had read only six books from. The idea was to bold the books you had read, repost the list as a note, and tag your friends to pass it on.

It was an arduous way to passively brag about how well-read you were. Dragen, a CS grad and website developer (he at one point designed the site for the rapper Tech N9ne), took what he learned building his first app. That one was called The Perfect Gift Finder, and it used friends’ Facebook Likes to suggest gifts for them based on the Amazon API. It flopped, he said, because there was no viral aspect to spur adoption. (Or perhaps he was just early. See now: Wrapp, Gift.ly, et al.)

But where he failed there, he succeeded with Appspring, the name he’s given his one-man operation. His list apps could not be more viral. Within the first few weeks of sharing the book list challenge with his friends, the app had reached more than one million users. The most popular, the Food List Challenge, has five million users.

Dragen said he’s been contacted by a few brands, including the Smithsonian, to make promotional apps for them. But he seems more interested in using the lists to support things he’s more passionate about, like Films For Action, an online news center that promotes social change documentaries, videos, articles, and posts.

Whether it evolves into a scalable business is up for grabs. It’s a content play, and a somewhat unsophisticated one at that. Dragen needs to continue adding lists to keep users returning, and now users can create their own lists. There are Christmas movie list challenges. Video game list challenges. Old time candy list challenges.

The possibilities probably go on forever; perhaps the novelty will fade out and it’ll end up in the meme deadpool. Or the idea will become diluted, or people will tire of the ads. Or someone could copy it. Or, with the help of a good designer, Dragen could parlay his impressive reach into an amazing catch-all “bucket list” kind of app that helps people track their experiences and aspirations. He’s at the inflection point now; I’ll be watching to see where he takes it.