I can think of many reasons why I wouldn't want to start a company, but a fear that one day Facebook will show up and threaten to copy me unless I accept a multi-billion dollar acquisition offer is not one of them.

Firstly, those offers will shrink over time as Facebook refines its playbook, and it becomes more and more clear that they can win by copying. Secondly, it doesn't just impact you the founder, it also impacts VCs thinking about funding you. $1-2 billion may be a lot of money for you and me, but if you're a venture capitalist investing $10 million in lots of low probability bets, you need them to have the potential for very high returns.

Fantastic^

The counterexamples: Facebook and Microsoft exist, yet Slack is worth over a billion dollars.

Facebook, Microsoft and Cisco exist, and Zoom is over a billion. I can't find a valuation for Bluejeans is doing, but they're no slouch either (they've raised at least 100M).

Google tried to kill Facebook this way, and failed.

As I mentioned before if your product is so weak at differentiation and defensibility then it doesn't deserve to be launched in the first place.

Facebook can only compete with you if you let them.