what i want to say is that, these sites r as u use them. if u use them properly they r of great help and if u misuse them, u r in great trouble.
look at me. i only add friends on facebook whom i already know personally. i dont add strangers. do not like spam and scam pages and do not host my personal stuff like my photo galleries there.
if u r cautious urself, there is nothing one can do to you.
And one more thing, dont trust a service unless u r paying for it.