Arguably, the combination of centralized and decentralized parts ace much closer description to what the internet is today than the idea that it's solely centralized. Same is true of society; while the 20th and 21st century are replete with efforts to completely centralize society under one organization (there is the Party, only the Party, and nothing outside of the Party), such structures have consistently failed and collapsed. (Which does not stop people from continuing to propose them seriously, unfortunately.) Society is a mixture of centralized and decentralized elements.

The problem is that in biology, you don't have intelligent entities exploiting the centralization, which is I think where the metaphor breaks down.

Interestingly, the metaphor isn't completely broken; I have seen serious discussion of thing like cancer, and even more interestingly, what can convince a cell to not be cancer, in terms of self-interest and exploitation and such. One of the mysteries of how multi-cellular life ever got started was how one cell convinces another to not seek its own survival at all costs. Being rather complicated multicellular entities ourselves, we take this for granted, but it's a legit research topic.

Still, the cancer abusing the system is not doing it with human-level intelligence, whereas arguably Facebook is a super-human level intelligence, so the metaphor is not great.