In terms of the Facebook product one cannot deny the obvious: They touched THE nerve on the internet. World wide. Across languages and cultures.
I don't like the embodiment of the product at all. At the very least it has usability and privacy problems. Yet more people use it successfully than any other web app in the world. So, what do I know? What do experts know?
A similar thing could be said about CraigsList. It's 2014. Every time I use the site I cannot believe what I am looking at. Yet I and lots of other people keep using it. It works.
Facebook has a general lack of elegance (whatever that means). The kind that happens when a product is thrown together and evolved over time. Evolving anything over time means the output "naturally selects" (subverting the theory here) to the environment created by it's users.
They survive because they optimized for what is important to their users. Grandma couldn't care less about UI issues or searchability. She wants to see her grandchildren's pictures and videos. And for that it works very well for a huge percentage of the planet.
At some point it becomes almost impossible to break the mold and clean-up what might be less than ideal. Why would you? It works. Another "Innovator's Dilemma"  situation to a large extent.