This one time, Zander Furnas and I were at a bar and were trying to figure out how to prove that someone had bought followers for their Twitter account - the original impetus was that we wanted to prove someone we knew was a charlatan who would totally do something like that. What we ended up with was a rough outline of an algorithm. I went home and wrote it up, tested it against accounts, and we let it sit. A few months later, Mitt Romney's Twitter account surged with many new followers, much more than you'd normally expect even during a presidential run. We decided to put our algorithm to the test, and were pleasantly surprised to see strong results. Read more about this over at The Atlantic.
To read more about how I implemented code for this prior to the article for The Atlantic, check out this post about detecting cheaters on Twitter