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It’s easy to fork the code but not the user base.

yes, when was the last time you see google chrome forked and adopted a large user base? That is event with big VC money behind it Brave for example. Open source is now days used a marketing scheme to attract developers to work for free, and attract user to think that is open.

There are a "billion" forks in the crypto space, but have any surpassed the original project in quality or adoption?

Sushiswap did for a bit, iirc. It's doing quite well either way, though.

There are hundreds of open source projects that implement a social network.

So does that mean I can be the next Facebook with one click of a deploy button ?

Facebook wasn't the first social network. Not even close. Facebook succeeded because it did very similar things to the prior ones, but a little bit better than them in just the right ways. That is the essence of a fork.