A study by Stephanie Brown at the University of Michigan found that women who performed "closeness tasks" with a partner had elevated levels of progesterone in their saliva. Furthermore, a week later, the women with more progesterone in their spit were more willing to make sacrifices for the partner.
The study is "Social closeness increases salivary progesterone in humans."
According to the LA Times Blog, the study used progesterone in the saliva as a marker for oxytocin in the brain. Other human studies assume that increased levels of oxytocin in the blood correlates to increased levels in the brain; saliva tests for oxytocin are still being developed. Not having to draw blood makes these tests much easier for the researchers.
It's notable that the later coooperation task was playing a cooperative computer game, in light of some recent editorials saying people needed to get off Facebook and onto face-to-face.
According to the LA Times, in a press release, Brown said,
Science Central has a good description of the experiment.
The bottom line: For women, hanging out with friends -- even if you're on the computer -- is really healthy, and makes you a better person. The same thing doubtless holds true for men.