Some of it could also be a plain Darwinian numbers game. Facebook was neither the first nor the only social media of its kind. There were hundreds of failed attempts at similar social media, counting those that died in obscurity.

When Google attempted their Facebook clone, it was just one of the many who took a spin at the wheel. It was always more likely to have failed than succeeded.

Building a B2C with hysteric adoption is difficult because it's very mysterious what elements of the product will actually lead to success, because it's a psychological thing. E.g., if Facebook chose green instead of blue as their theme color (all else equal), it might've died in obscurity.