Some friends of mine I know better by their nickname, or even only by their nickname. Once a friend of mine called me in her capacity as staff of an organisation I volunteered for, introducing her with her formal name. Took me a moment to realise who I had on the phone.
Both my parents use different first names in daily life than the names listed in their passport. Everyone knows them by that name, using their official name would cause serious confusion. My mum has four given names, a maiden name and my father's name all listed in her passport. If Facebook insists on real and complete names, that'd be a practical problem. It's almost 50 characters in all.
My wife has formally only a Chinese name, but in daily life uses an English name. Common practice for Hong Kong Chinese - some opt to have their English name (often adopted during primary school, not given at birth, though the latter happens more and more nowadays) registered on their ID card, others don't. She also doesn't have her English name registered. So while most call her by that name, only some use her Chinese name, it's still a nickname.