I too have been reluctant to sign up due to privacy concerns. I saw a documentary regarding Facebook where it was explained that by signing up you grant them access to see your browsing history even when not connected to the Facebook site. I suspect that by now there are ways of opting out of this "feature," but does Facebook provide an easy and obvious avenue to do this? Incognito/private browsing is always possible, but it's a PITA signing-in to benign sites (like QQ) every time one visits.
It's the home of numerous security/identity theft problems...it's getting harder to protect your identity anywhere on the internet...not really aimed at facebook...but one of the newer forms of obtaining your financial information is actually on legitimate sites...It's called formjacking...and my SS number has appeared on the dark web as verified by 2 different sources....you might want to check on your SS number so see if you have been IMPACTED...I have taken various security measures over the past few years(lifelock and checking my credit scores and credit reports) but the reality is that the cyber criminals are winning this battle and you have to be aware of your credit dealings...
On the social side of facebook... I just don't get the fascination people have of knowing every mundane activity of other people...the only positive thing I see is connecting with people from your past that you have lost track of....now that would be beneficial....to each his own