What if, that social network was built by 5 people, college aged, and they decided to use FB login to save some time developing because they are trying to see if this thing is worth building. A very real probability.
FB won't be the powerhouse that it is today, but it's deeply entrenched in our internet that there is value in the social data, not the "blue room" product they offer for free now. (I hate FB, but I get it.)