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I think the key part is "if facebook disappeared and was replaced by another tool." The important part is how many people are in the SAME tool. The value to the user of a network depends on how many other people are in that network. The same number of people, split up into different networks, wouldn't be a linear decrease in value, it would be an exponential decrease in value.

Maybe the fact that reality is clearly contradicting your intuition about value is important? 1 Billion-ish people find Facebook valuable. You disagreeing with them just makes you wrong, or maybe lacking in empathy. I didn't understand the value in ridesharing. I still think it shouldn't be nearly as popular as it is. But that's me not understanding reality; it isn't reality being wrong.