Do you trust your family?
Do you trust your friends?
Do you trust your close work colleagues?
Do you trust the rest of the staff?
Do you trust their family and friends?
Let us think about how trust is open to abuse.
You are a fairly regular user of a social network. You get an in-network message from a friend who has problems with their account and is asking to be your friend again. You have already added them before but are happy to add them again, after all ... if you ask them to prove who they are it would just make you sound paranoid. You go into work the next day to find out that pictures you shared with close friends of you as a young adult have been printed out and shared round the class ... from the summer holiday which you barely remember (images of Bez talking about banging tunes and 'avin' it could be appropriate here) and you with hardly any clothes on or surround by others with hardly any clothes on ... and then there is that stag/hen weekend you went on with friends ...
Yes, the chances of it being someone faking a friends account are small, but just think about all the photos of *your* friends you might be giving access to others.
A member of your family will have your phone number or email that they can use to let you know it is a legit request.
A close friend will appreciate you protecting *those* photos from being seen elsewhere.
Friends at work don't want their moans and groans being aired to the wider world.
When you invite someone in or grant someone access they should be a person you know is real and can trust. Once you have explained the rules about how you want your pictures and facts treated then they should respect this. After all, if you loaned someone a physical photo album you would not expect them to make copies and leave it lying around. If you lent someone a treasured book you would not expect them to mistreat it or allow their children or friends to scribble in it. If you lent someone clothes you would expect them back in a good condition. We have always made rules with one another about how we expect others to treat us, our treasured goods and our friendships ... by letting others know these rules about how you want to be treated online then they have a choice. Follow it or lose your friendship. Asking someone to prove who they are is not being paranoid ... it is being safe. Trusting someone you cannot be 100% sure about is foolish, and if you do that with your online identity then you risk things being abused.
The same social engineering tricks are used by scammers, computer abusers, spreaders of viruses, and those up to no good. Don't get caught out, enjoy being online and be safe.