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Their key finding was that the total number of mutual friends two people share — embeddedness, in social networking terms — is actually a fairly weak indicator of romantic relationships. Far better, they found, was a network measure that they call dispersion.

This yardstick measures mutual friends, but also friends from the further-flung reaches of a person’s network neighborhood. High dispersion occurs when a couple’s mutual friends are not well connected to one another.

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Their dispersion algorithm was able to correctly identify a user’s spouse 60 percent of the time, or better than a 1-in-2 chance. Since everyone in the sample had at least 50 friends, merely guessing would have at best produced a 1 in 50 chance. The algorithm also did pretty well with people who declare themselves to be “in a relationship,” correctly identifying them a third of the time — a 1 in 3 chance compared with the 1 in 50 for guesswork.

Particularly intriguing is that when the algorithm fails, it looks as if the relationship is in trouble. A couple in a declared relationship and without a high dispersion on the site are 50 percent more likely to break up over the next two months than a couple with a high dispersion, the researchers found. (Their research tracked the users every two months for two years.)

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– http://bits.blogs.nytimes.com/2013/10/28/spotting-romantic-relationships-on-facebook/?_r=0