I actually don't worry that they'd screw with the money at all. They are professional & big (think: Won't go away, can pay from their pocket if they screw up) enough in my book to trust them with transactions.

Unless you consider that you're now not only giving them data about your private life, friends and preferences, you add the 'successful conversion, bought X' data point to a data set that is already too big by my standards. It's like combining my address book, private pictures, credit card bills and entering that data into a huge corporate database so that I can chat and plough a virtual field.