Either it's private to you, in which case this service is very likely not going to provide you a great deal of value, which puts you outside their market, or it isn't private, in which case you're in their market.
>Seriously, would anyone approach potential clients with that kind of attitude in real life?
This is real life. I don't like the implication that "it's an internet service" means "it isn't real life." This is a service that's trying to land you with real people, doing real things, in a personal way.
The attitude here is "you're looking to be social, so sign up with your social network." If you don't like that, then I'd warrant that you aren't the type of person this is aimed at.
It'd be nice to have alternatives -- others here have suggested a survey, for example -- but let's be honest: Facebook collates a lot of really useful information that is directly helpful to a service such as this. It's a no-brainer from their point of view.