Yes at a simple level you're right, but you could use the same argument about lots of things, for example I could opt-out of using the Internet but that should not give ISPs carte blanche with my personal data.
As to your question, I use FB fairly cautiously and I'd also rather not exclude myself from a social forum which friends prefer to use.
What I'm suggesting is a simple compromise, rather than give social networks an unlimited license to my data I'm happy to give them a more limited one.
If you're actually concerned about people destroying your data after six months, why are you giving it to people lacking such an agreement?