Biggest perhaps is the more than one time they reset people's sharing from friends only to public.
But you don't have to accept every friend request you get. And every post can be one of several sharing levels.
Of course, FB should never be considered secure, any more than anything else you post on the web. People should never anywhere online talk about an upcoming trip, only talk about when they get back home if they wish. Things like that, never giving clues when you're going to be gone or home, or talk about new expensive purchases, etc.
But it can be used without much more risk than anything else online if you thing about what you're doing.
My niece wants me to be on FB. I tried FB when it first began for a year or so and didn't like it at all. Friends, friending friends of friends and friends of friends friends, it was mind boggleing . I was aquiring this hugh friend list of people I didn't know and they were getting to know or be aware of my personal life . Something just didn't feel right about that so, as John Lennon said in a tune "I just had to let it go"! I do just fine talking to relatives over the phone and e-mail but I don't want to publish my life. "Big Brother" (the NSA) is keeping track enough of what where I'm going and doing and saying without me giving it up freely , they at least have to work for it. I know the whole world can see what I'm typing here on this forum, but for some reason it just doesn't seem the same as FB. Is it the same?.....Digger