they are as safe as the information you put on them.
I personally use a throwaway email for facebook, with an inbox i never check.
My details are half truths, enough for friends to find me only, but nothing completely true, even my phone number is only the last 6 digits, so people can check they have the right one.
Treat the internet as an open book, and never type information anywhere which you do not wish anyone to see.
The fascination with posting all your details, bra size and how often you trim your toenails eludes me.