Last week, I enlisted Slate readers to help divine how Facebook's "25 Random Things About Me" trend got started. More than 3,000 of you responded, answering queries on when you first saw a "25 Things" note, when you were first tagged, and when (if ever) you wrote your own note. On one level, the survey was a failure: I had hoped to find the trend's Patient Zero, but there's likely no single person who conceived of this scheme. But the absence of a singular "25 Things" creator reveals something much more interesting: Facebook organisms are not created by intelligent design. They evolve.
The idea that culture spreads in biological ways has been around for a while. Richard Dawkins coined the term meme in 1976's The Selfish Gene to describe how ideas propagate according to evolutionary principles of mutation and selection. A quantitative study of the "25 Things" letter seems to ratify that.
As many readers noted in our survey, "25 Things" wasn't always "25 Things." Late last fall, a chain letter titled "16 Random Things About Me" began to chew its way through Facebook. The author of one of these notes would itemize her personality into "16 random things, facts, habits, or goals," then tag 16 friends who would be prompted to write their own lists. And so on and so on. Similar navel-gazing letters had popped up over the years through e-mail and on blogs, MySpace, Friendster, and the venerable blogging site LiveJournal. The Facebook strain had a good run, but by the end of 2008 it appeared to have stagnated.
Then something curious happened: It mutated. Since everyone who participates is supposed to paste the original instructions into her own note, it's easy to tinker with the rules. Soon enough, 16 things (and 16 tagged friends) morphed into 15—and 17 and 22 and 35 and even 100. As the structure crumbled, more users toyed with the boundaries. Like any disease, "Random Things" was mutating in hopes of finding a strain that uniquely suited its host. In this case, the right number was vital to its survival: The more people who are tagged, the more likely the note is to spread. The longer the list, though, the more daunting it is to compose and the fewer participants will be roped in.
By mid-to-late January, "25 Random Things About Me" had warded off its competitors. Once the letter settled on 25 things (a perfect square, just like 16) the phenomenon exploded. The data we collected reveal a clear tipping point around this time.
As the graph below indicates (Fig. 1), the number of people swept up in the trend climbed steeply for a week starting around Jan. 20, peaking in the last days of the month before declining sharply. Not coincidentally, the Web analytics firm Compete reports that January 2009 was one of Facebook's biggest months for traffic growth.
A graph of when people wrote their own 25 Things note (Fig. 2) forms a very similar curve.
Since I'm no evolutionary expert, I shipped Slate's data to Lauren Ancel Meyers, a biology professor at the University of Texas who models the spread of infectious diseases mathematically. Meyers says that around Day 39 of Fig. 1, we see the "classic exponential growth of an epidemic curve." (Day 39 in this graph is Jan. 8.) She also explains that "25 Things" authors can be seen as "contagious" under what's known as a "susceptible-infected-recovered" model for the spread of disease. Think of "25 Things" authors as being contagious for one day—the day they tag a bunch of their friends. Meyers found that, for that one day, the growth parameter of the "25 Things" disease during its ascent phase (roughly until the beginning of February) was 0.27. This means that, on average, each "25 Things" writer inspired 1.27 new notes.
Another one of our survey questions considered the average number of days between when a person is tagged and when she writes a note. Those results are graphed here.