Facebook IS the biggest photo sharing site.

End up? That's basically where they started. The popularity of Facebook in its very early days was because they made it easy to "stalk" your friends (and/or crushes) via the photos that they shared.

If Facebook hadn't made photographic integration so central to their platform, I don't think they'd have taken off the way they did.

That's also why they were so scared of Instagram - they knew how powerful a network of photo sharing can be.