For their unpublished study, titled Gaydar, MIT students Carter Jernigan and Behram Mistree scanned the Facebook friends of over 1,500 fellow students. They've found that homosexual men have proportionally more gay friends than straight men. From there, it was easy to create a piece of software that can predict the sexual orientation of Facebook users who haven't disclosed it. Simply put, if you're a man, and you have a large proportion of gay friends, you're probably gay, and you can't hide it.
The results of the study, as always, need to be taken with a grain of salt. The program correctly guessed the sexual orientation of 10 men who were known to be homosexual, but did not explicitly reveal it on Facebook, which is an awfully small sample. Furthermore, the Gaydar didn't correctly identify lesbians or bisexuals.