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.