As FindLaw's headline for this item suggests ("Despite Facebook Stabbing, Couple Stays Friends"), this actually ended reasonably well, as these things go.
Reportedly, the argument was caused by a 22-year-old Indianapolis man's refusal to let his girlfriend see his Facebook page. The report did not say why he was reluctant to do this, but the number of options is probably limited. The argument, which had begun upstairs, then moved downstairs, "and it was then that McVey says she accidentally slashed Davenport with a knife that she got from the kitchen." (Accidental slashings like these are all too common and could be prevented if manufacturers would stop making these defectively sharp kitchen knives.) Ultimately neither party was charged, and according to the report, "[t]he two remain Facebook friends." So maybe whether they remain actual friends is not entirely settled.
The growing importance of social media may or may not be indicated by the fact that two of the last three items in this category have involved Facebook. See "Good Reason to Kill #10: Posted 'Ugly Baby' Comment on Facebook," Lowering the Bar (Sept. 20, 2010).