>>aiiane+h6
Perhaps because Facebook provides an easy and fairly reliable way to determine someone's sex. Many projects including dating sites have relied on it to do authentication for this reason.
>>EGreg+D6
A sizeable and growing portion of the population refuses to have any dealing with this shady corporation. Facebook is not a universal authentication platform and never will be.
>>glorkk+Y9
There isn't a universal authentication platform. Facebook is closer than most. And Facebook is strong for this in the interest of getting something out the door, to be improved later.